I'm not sure why you find it so incomprehensible that a person might infer from the actions of another their views on a given issue. Perhaps you think that people's minds and bodies function independently of each other? That what pro-lifers think and what pro-lifers do belong to two completely distinct and non-overlapping spheres of being, with no possibility of inferring the one from the other? It's possible that this is the case, and more power to you, but it's not something that you have any good reason to expect from the rest of us.
I think this might be where we are diverging. We are admitting that if the world turns away from them, then they don't exist. There is no external force to dictate them outside of ourselves. However, if we "act like they are true anyway" then they do exist, significantly and measurably.
Bear with the bad analogy please - It feels as if you are saying "Football has no rules, they don't exist."
What you're describing here is legal rights, though, not natural rights, and that's not what the abortion debate is actually about. If it was just a case of establishing whether a foetus had a legal right to life, or a woman has a legal right to bodily autonomy, we'd simply consult a lawyer working in the appropriate field and he would clear everything up for us in a jiffy. So clearly this is about something more fundamental than that, something which we do not claim to construct but rather to reveal, and my contention is that these alleged revelations are baseless.
That doesn't stop us from acknowledging the existence of legal rights, or from working within their terms, but it does have implications for how we engage with them. We can no longer say that such-and-such group
has a set of rights, and then demand that this be legally recognised, we can merely say that we think they should be accorded it. Nobody has an intrinsic right to free speech, for example, we simply think that it is a good idea to accord it to certain people in certain circumstances. It becomes a simple question of utility. But that this isn't actually how the discourse is conducted, even among those who explicitly style themselves "utilitarian".
(Now, speaking for myself, I think that the fundamental problem with the ethical language of "rights", natural
or legal, is that it poses as being fundamentally unfree, able to move only within the terms offered to us by these "rights". I regard that as absurd, as completely contrary to the obvious facts of the matter, which is that we are in ourselves completely and utterly free: that any restrictions on free are imposition, by ourselves or by others. We do not need to say "I have a right to free speech", but rather that "nobody has a right to prevent me from speaking".)