Two points.
You are not making points. You are issuing challenges.
Why is this any less arbitrary?
I'm not sure I understand the question. You are not asking why I would value my own interests, are you?
So if I handed you a baby and offered you $10 to kill it, assuming no one else would ever find out, would you do it?
I respectfully decline the challenge.
Actually I don't think infanticide is as universally evil as its cracked up to be.
Don't get me wrong judging by my personal ethics infaticide is wrong and once we will have artificial uterus's and genetic therapy abortion will also be wrong on about the same order of magnitude.
But going by general human behavior rather than specific ethical systems both infanticide and abortion where generally accepted far more than today. Not only that the specific notion that something magical happens when a organism passes through the birth canal making it worthy of moral consideration seems to be a very modern and Eurocentric thing.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.