That wasn't setting up religious people as irrational. Its perfectly rational for people with those basic beliefs to follow their doctrine on the matter, but I personally reject the doctrine. Rationalistic was to denote a belief in rationalism: "a belief or theory that opinions and actions should be based on reason and knowledge rather than on religious belief or emotional response".

Still, you implied that one with a belief in rationalism could not come to the conclusion that life begins at conception. If that was not your intent, then I'm fine with this.

They cant live on their own and have yet to do anything but have potential. I feel like I moved my own goalposts a bit, but the point remains that are fully dependent on another human being to live, until viability.

This is getting a little questionable for two reasons. The first is that even a baby cannot live on its own, and we already have laws that forbid a mother from abandoning her baby to die. Caring for a baby still requires an expenditure of bodily resources, even if it's a different set than carrying a baby.

The second reason it's questionable is that viability is a moving cutoff. As medical technology gets better and better, fetuses will be viable earlier and earlier. In fact, if artificial wombs ever become a thing, all fetuses will be viable.

man-slaughtered then, could be argued with that definition of personhood.

Ah, but then it could be applied to miscarriages, making this an argument that abortion should be unrestricted. A position that can be held, but is not very popular. It also implies that all SIDS cases should be treated as manslaughter, which is generally not agreed upon.

Alternatively, one could deftly dodge this issue by saying life begins at implantation. That's not a sentiment I've seen much, but it does have merit.

We may need to end up agreeing to disagree here, but I do think this discussion has been productive.