I'm not denying the importance of treating something as sacred as a way to regulate human behavior. (I do deny that as a policy matter, what gets to count as sacred gets to be determined by one particular religion--that is not appropriate in a country with separation of church and state.)

If you pick one definition and it causes actual measurable harm among people whose humanity is in no doubt whatsoever, it is entirely reasonable to ask: well, was our arbitrary definition nonetheless reasonable?

You can appeal to the profane reality of the situation, but that tells you: conception is really cool, but that's not where the special characteristics of being human actually start. Eggs and sperm have tons of potential, extremely little realized. A fertilized zygote has had a tiny bit of that potential realized and that's all.

If we define that as the boundary between sacred human life and not, and cause ourselves a bunch of problems, then the problem is with our definition--we picked a bad boundary to distinguish the sacred from the ordinary. Pick something else, like cortical activity, or the ability to survive outside the womb, or something--try those out and see what the downsides are there.

And why does it need to be a boundary, rather than a gradual phasing in? I agree that it's probably easier psychologically, but is it actually necessary to have a sharp boundary as long as all the important cases are clearly in? Why can't it be fuzzy but still allow us to retain our humanity?