This is a general thread response. I have read a good number of the posts and synthesized the general points. The following will be presented in numbered sections.
1.1The moral status of a fetus
A logically consistent atheist cannot believe that something having human DNA and a metabolism somehow makes it worthy of any sort of moral consideration. At least not directly (more on that in a minute). This is because human DNA is not in any way special, the only thing separating it from the DNA of a chicken, lizard, or brewers yeasts, is the nucleotide sequence. Obviously, there are no intrinsically superior nucleotide sequences or anything like that in a universe in which there is no design.
What is really under discussion here is whether or not the zygote is a person. Which it is not. It has none of the properties of a person that would make it have worth under any coherent ethical system. In fact, over 80% of them will be miscarried anyway, most of them without mom ever knowing she had a zygote inside her.
Now, personness is not even all that important in the broad scheme of things, fundamentally for the same reason that human DNA is not a criteria.
What matters are general properties, and these will depend on what ethical system you are applying. I will go through the three biggest ones.
Natural Rights: In most formulations, the fetus has rights at some arbitrary point depending on the formulation. However, in no formulation of rights ethics is violation of someone else's rights (generally considered the initiation of force in the case of rights regarding one's physical person) acceptable. The mother did not strictly speaking ask the fetus to be there. She owns herself, and in just the same way as she can refuse sex even while during the act, and back that up with lethal force to defend her right to control her own body, so too can she use lethal force to expel the fetus, even if she had given it prior permission to exist. By it staying against her will it is initiating force.
Kantian Deontology: The categorical imperative requires that every sentient being (basically those that can pass a mirror test) must be treated as a end unto itself and not merely as a means to an end. A child cannot pass a mirror test until around the age of two. As a result it can be treated as a means to and end and be disposed of at will. There goes the fetus.
Utilitarianism: I will consider here two variants. Hedonistic and Preference. In hedonistic utilitarianism, Good is measured as the ratio of pleasure to pain or suffering, and is weighted by the capacity of the being(s) in question to experience either of these states. A fetus cannot feel pain, or experience pleasure until it is very late term, and its ability to comprehend these (and thus foresee future suffering or experience existential angst via fear) is very low even well into early childhood. By comparison the suffering of the mother if she does not want the fetus, and foresees a considerable degree of angst raising the child, is very high. Third parties have a decreased magnitude of suffering if they are opposed to abortion, consisting basically of being annoyed or outraged at an abstract concept, and they are largely canceled out by those with equivalent but opposite views on abortion. Thus, under hedonistic utilitarianism, the fetus has no direct expectation of having any more moral consideration assigned to it than we would attribute to a rock or chicken egg, and the mothers suffering will probably outweigh the net effect of the indirect consideration granted to those who oppose abortion.
By contrast, preference utilitarianism defines good as the ratio of realized preferences to unrealized preferences, with each preference for stakeholders weighed according to the strength of their held preferences and the relative effect that the consequences of the decision have on them. A fetus cannot hold a preference at all, because it lacks a mind. By contrast even a lizard can feel fear and obviously would prefer not to die. Mother obviously holds a strong preference and the decision impacts her greatly, while the strength of the preferences of third parties may be high, the decision does not impact them at all. This is because there are an infinite number of children that could potentially be born, and there is no way of knowing whether the impact on any one third party will be positive or negative. Think of it this way. Say your mom had a 2nd trimester miscarriage. Do you miss your unborn older brother? No. You dont know the difference.
As a result, there is no case to disallow abortion under any of these ethical systems.
1.2Obligations to Future Individuals
It can and has been argued that abortion should not be permitted because we have an obligation to secure the happiness of future persons, and this also implies that we have a responsibility to make sure that those future persons are born. This argument is a non-sequiter. While it is true that we have obligations to secure the happiness of future persons, it does not follow from this that we have an obligation to secure the existence of particular individuals. There are a number of reasons why this position is not tenable, and I will only give the most important one.
If we follow this line of reasoning, it would follow that we have the obligation to secure the future existence of all possible persons--by which I mean to say that we would be obligated to secure the existence of an infinite number of possible persons. A logical consequence of this is that no one could refuse heterosexual sexual intercourse because it would eliminate the chance that a person could be born from said sexual union. Certainly this position is completely absurd. As a result, it is better to say that we have an obligation to secure the future living conditions of future persons, but not to secure the existence of said persons.
2.1Obligations of Sexual Partners--Specifically addressing the main thrust of Seth's argument.
Something has come up in this thread regarding a proposed disparity in reproductive rights between men and women. This person, Seth proposes that the current system whereby women are permitted to abort a pregnancy and men are not, and whereby men are stuck on the hook for child rearing is unfair. Seth also claims that he is a nice guy who finishes last. The way he talks about women, I have concluded that this is [probably not true. I cannot be certain, but his rhetoric is that of a misogynist. If he does finish last is because no woman will go near him with a three meter aluminum rod after the second wave f feminism in the 1960s and 1970s. Rhetoric about how oppressed men are when they are forced to take responsibility for their profligate philandering is just not in vogue this century. However, to base my argument on this would be fallacious. Let us examine the fundamental issue of fairness.
Men have sexual intercourse for a number of minutes, ejaculate, and then—other than the risk of disease which per incident of intercourse is lower than women—bear no further biological risk of pregnancy. Depending on whether the fetus is brought to term, they may suffer financial risk if they do not wish to have the child. At most, barring the death or unfitness of the mother, the father need only provide 50% of the social aspects of child rearing, and often less.
Women have the potential to die in childbirth, and suffer a number of complications due to pregnancy both before and after they bring the child to term. The mother can abort the fetus, but if the fetus is brought to term will provide between 50 and 100 percent of the actual care of the child.
So, in short:
Father
0-50% child care
0-100% financial support if he sticks around
0-”50%” (it is often less than this depending on child support agreements) if he does not
If abort: none of these, except for being sad for a few weeks
Mother
50-100% child care
0-100% financial support
Extreme biological risk
If abort: None of these
If forced abort: None of these, but has basically been medically raped.
If denied abort: All of these, plus suffering due to having to carry a fetus she does not want to term, and then having the majority of the burden of raising it.
If we are to talk about fairness, who here has the greatest risk, and thus should be the one who primarily makes the decision? Who has the most to lose, essentially? The woman does.
Seth's argument is completely and utterly incoherent. It is disconnected from a place we call “reality”. If one wishes to talk about “taking responsibility for actions”, abortion is doing just that. But as Seth so well put it, life is not fair. Women have an option available to them by nature of being the sex which gives birth. Termination of the pregnancy. Men do not have this. To give men this option would by extension permit us to force other surgical procedures on people as part of an implied contract.
Seth phrases his argument in terms of “The woman wanting cock”. However, it is seldom the case that women do no have to be convinced to have sexual intercourse. Usually the male has to engage in some sort of display before he can mate. Much like the amphibians I study:
(Please disregard that both of those frogs are male)
The proper phrasing should be as follows:
“Guys, if you want pussy, you accept the risk that you will have to take responsibility for a child that the girl you banged wants to keep.”
Seth's argument sounds like the rhetoric of a petulant teenager, desperate to not have to take responsibility for their actions, and trying to lay them in whole on someone else. It takes two to tango. Biology and basic medical ethics have given women an option Seth does not have which seems to be one of the few fair things that exists in this universe. He has spent this thread winging about it, and casting about for ways to make himself, as a male, appear oppressed.
2.2Obligations to a Child
The above treats the matter of male and female rights as a two-body problem, only dealing with the moral issues arising from two persons. When and if a child is born however, a third term enters the moral calculus. The child.
It does not really matter what ethical system we choose to utilize. I tend toward utilitarianism, but for the sake of brevity, I will stipulate that this conclusion follows from all three of the major ethical systems I discussed in section 1.1. parents have a duty of care to their child. Moreover, all adults have a duty of care to children. It does not particularly matter which adults care for the child, but the responsibility of ensuring that someone does, falls to the biological parents. They took the set of actions which brought the child into the world, it is their responsibility to ensure that someone cares for it. Generally, that is them. This means that the biological father must at minimum financially support the child. This is the basis for the “best interests of the child” doctrine in US family law. Thus, the father cannot abdicate his responsibility unless someone else takes it over completely (adopts the child) thus transferring permanently the responsibility in a way less transient than even becoming the child's stepfather. If a divorce happens, the stepfather's responsibility toward his step-children ends.
So, what about cases of false paternity? Yes, there was deceit, or more likely, the mom just did not know. I can go into the complicated evolutionary biology of this (sperm competition etc) but I will not. The point is, the presumed father took initial legal and moral responsibility for the child. Presumably, they formed some sort of emotional bond. In these cases, the biological father did in fact fulfill his responsibility to ensure that someone takes care of the kid. The deceit inherent in cuckoldry is, with respect to the child, irrelevant. It is in the best interests of the child that the person who initially began caring for it, and whom it formed an emotional bond with continue to do so. The suffering faced by the child being abandoned by a parent they love and depend on outweighs the feelings of betrayal felt by the cuckolded father figure. They accepted this risk as well, when they began having sex with the mother.
Winging about it, and bewailing the unfairness of the situation does not change the moral responsibilities that fall on all parties involved.
I'll return to the thread later to respond to your posts, when I've chewed them over in my mind a little to come to a way of articulating my points, and I'm no longer ready to pull my hair out over the apparent communication failure. 
