I would never marry anyone who wasn’t poly. The law is the one making assumptions here.

My point is that it shouldn’t be possible for my spouse to persue criminal action against me for fucking someone else in a poly relationship if we’re married. Thats fucked.

The law shouldn’t make it not allowed to fuck someone outside your marriage by default. As I said before, thats cool to make it optional like a prenup, if you’d like, but it shouldn’t be the default