I wish it was possible to make gay families xD the line wouldn't grow anyway lol but doing so I think it would add some "challenge" 'cause if you do match the couples too randomly and too fast you could get someone who can't physically produce children together, as well as dying like it is now. (= adds more possibilities to dont be able to make the family tree growing, lol)
Or, it would be fun to add the possibility to cheat aha like adding a third person to the couples... illegitimate heirs! xD