I don’t think there’s just one “line” in this situation. It’s “OK”, in the sense of “ought to be legal”, to kill a fetus at many different stages of gestation, for different reasons. Arbitrary lines are drawn because we need them for purposes of legal consistency, not because they are logical or sensible in terms of biology.
This means, by your own frequently expressed claims of moral conviction, that you are okay with occasionally murdering fully human persons as long as the murder is just a backup plan instead of the primary intention.
Morally speaking, that’s really no different from saying that to prevent pedestrians from walking down a particular street you should put up barriers at the intersections and also lay landmines to blow up the occasional pedestrians who happen to go past the barriers. Although the primary intent is not to murder human beings, the strategy is deliberately chosen in the full knowledge that it will sometimes murder human beings.
The fact that you would never consent to such an approach to traffic control, but are fine with birth control methods that occasionally kill fertilized eggs, is just one more demonstration that you don’t really consider fertilized eggs to be fully human persons, no matter how much you want to believe you do.
But as this birth-control discussion and many previous points in the thread illustrate, your logical reasoning pretty much sucks. It is simply not logical to claim that you consider a fertilized egg is a fully human person and also to condone sometimes killing fertilized eggs for no other reason than that their existence inconveniences you.
The same objection applies, in spades, to your earlier and equally illogical assertion that you have no problem with killing “fertilized eggs in test tubes”, because “non implanted eggs are non-potentials”. If a fertilized egg is a fully human person, then a viable fertilized egg in a test tube is no more “non-potential” than a human being in an airplane or an elevator or any other artificial environment where they couldn’t survive for a prolonged period without support services. If you wouldn’t condone killing people in an airplane but you don’t mind killing viable fertilized eggs in a test tube, then logically speaking you do not sincerely consider that fertilized eggs are equivalent to fully human persons. No matter how much you may want to believe that you do.
It may be obvious to you, and as I said, there’s no reason you can’t maintain that as your personal belief if that’s how you feel about it. But if you want to actually convince anybody else that it’s objectively an obvious or logical or sensible position to take, you will have to make some way better arguments than you’ve produced so far.
Well, of course it’s not logical: as I’ve been saying all along, it’s intrinsically illogical to assign arbitrary cutoff points to a continuous developmental process. We as a society assign such arbitrary cutoff points because we have to in order to maintain an artificially consistent way of dealing with human gestation in our legal system. Not because it makes logical or biological sense to do so.
Your own position, with all your wobbly thinking on whether or not fertilized eggs are actually equivalent to human persons and your arbitrary chopping and changing on when and how it’s okay to kill them, is no more logical than this one. The difference is that I’m perfectly willing to acknowledge the fundamentally illogical nature of trying to assign a single instantaneous “starting moment” for human personhood, whereas you’re heavily invested in not acknowledging it.