Crazy alternate universe, where males can only reproduce with males and females can only reproduce with females, but either pairing can produce children of either gender; this pattern extends to any species which does not reproduce asexually. Give careful consideration to social changes--e.g., like IRL each couple would need at least one partner to work to support them, back through history, so gender discrimination is not going to have ever been like it is or was here; also, everybody is gay is the only thing that makes sense here.

Protagonists are a straight couple in the modern day, extremely aware of how lucky they are to be living in a time when it's legal and to some degree accepted for them to be together and hoping that someday soon it will be legal for them to get married.