What have you tried? I don't remeber trying.
Viewing post in Same-sex Marriage
Nothing that out of the ordinary. Playing as a male protagonist, in Act 1 of the game, I wanted to marry one of the male slaves I'd had for a while. I tried to propose through the normal dating option, but "points out that it's not possible to marry someone of the same sex in current customs." It does occur to me that it may only be possible later, but I assumed that this is a Watsonian reason for a Doylist restriction.