In politics the BATNA will be different as there are real costs (like some bad, faith, actor is going to tell you what you can or cannot do with your own body).
That's a bit different from a bad faith actor or intent to deceive. There are entire branches of human endeavor (basically all the liberal arts, but here philosophy and theology are particularly relevant) built on discussing the immaterial or that without evidence.
To take your example of the abortion issue in America, your steel man argument has to explain why an acorn is the same as an oak tree, or perhaps the difference between an acorn and a fertilized human egg. They have to account for the mass murder of fertility clinics and researchers throwing out thousands of fertilized embryos per day.
Or you could choose to accept their philosophical argument about the start of life as a given, and discuss the consequences of it. E.g. you have a right to kill another human in self defence, unless the person threatening your life is inside a womb. Or the conditions under which you have a right to another person's life and body. Is there an innocence test we could do for organ transplant recipients, where the top end of the scale gives you a right?
Lots of room for good faith discussion there, even on immaterial subjects.
