Wolf clothed as a sheep. If biological sex is so important for personhood, then it is easier for the patriarchy to draw lines of value in that personhood. That means it is easier for men to continue to hold unequal power.
People are people period. Fully equal and fully deserving of equal rights.
Regressives are very afraid of blurred lines because it further upsets an eroding power base, it also to them anyway, calls into question sexuality. How can they whip the faithful up into a frenzy about fighting against gay rights because "they love the sinner" if nobody can actually define "gay" any more.