And also, that polygamy is cool on paper, but by law a man can only have one wife. Now you might think who cares about marriage, its only a piece of paper, but it goes way deeper than that.
A wife has legal rights a gf does not. Im talking providing health care for you, tax breaks for you, getting whatever is his if there is a tragic accident, or even worse case, getting half if shit dont work out, plus way more.
Im not saying he should marry you, if he doesnt marry either one of yall its cool, but if he gets married to her, and you just the gf, you taking the short end of that stick and would be stupid to put yourself in such a disadvantageous situation.