I think the player should be able to set which male they want to mate with which female (that is if they want a couple to be only with each other). I was trying to do that with my two 18yr olds and they had two kids together, but then a female who was in her last breeding year snuck off with the guy (who was 19-21 at that particular time) and messed up my plans. I think there should be a way to fix that if the player wants to, but also they could switch it if say one partner passes away (so the one left behind could find a new spouse lol).