Hmm. You know, I bet you could get more-or-less the same relationship patterns without the specific biology if you had any sort of variation on soulbonding? So everybody has a destined soulmate, and once you've found and bonded with your soulmate, you have to stay with them forever. But because a lot of people have same-sex soulmates and population replacement is an issue (and also because soulmate doesn't necessarily match sexual orientation) the society also enforces heterosexual marriages for people in same-sex soulbonds, and has a concept of non-sexual soulmate. (With space for queerness worked in wherever necessary.)

...that sounds entirely awesome, Sime-gen or not!