It’s silly to pretend that secular people don’t distinguish between human and animal life. Both religious and secular people see murder - the destruction of an innocent human being - as an abhorrent crime, where only a small percentage of PETA types see the destruction of a cow or chicken the same way. Our laws reflect that. If pro-lifers put more energy into pointing out that abortion is the destruction of a human being - a being with unique human DNA, not some amorphous part of the mother’s body - more people would be against it.
And I never said religious arguments were out of bounds. For people with some religious background, I’m sure those arguments can be effective. But for those who don’t come from that worldview, it’s useless to argue on that basis. And I think the pro-life movement has severely overplayed the religion card at the expense of other approaches.
I think secular people do distinguish between human and animal life, but the reason they do is based on convenience, not logic. Pigs, for example, are happy when you pet them. They squeal when you cause them pain. They eat, sleep, reproduce - all on their own. They are alive in every sense of the word. What makes killing them for food more ok than killing an unborn baby?
Again, I think it comes back to a threshold of convenience that each person has, which is entirely subjective and not based in logic at all. It is OK to kill animals for food because its an efficient way to get calories, and its ok to kill an unborn fetus because that kid may put a crimp in the plans of its parents.