There should be an option in friendly saying "Help with a Baby". Basically you can ask any random woman, not pregnant and if she'll accept, to go with one of your homosexual characters to the hospital for an hour or two and then they return with a 100% of pregnancy but only one baby will be born.
It should work the same way for a Lesbian couple as well, but with a random man, if he accepts, again it is to the hospital for a few game hours with the one women getting pregnant who went with the man.
Well maybe not completely a random male/female, maybe a checkpoint in the friendship relationship status needs to pass, say "Liked" which is about 35% into that bar? In any event the random woman who gives birth or the guy who passes the gift of life, they have no relationship with the baby.
I'd also go about restricting some men and women and give them a trait "Same Gender Romance", meaning they will only date and marry people of the same gender to themselves. i'm sure there is a way to allow other parts of the LGTBQ movement to have a role in the game as well. I mean you wouldn't want to see a girl hitting on a guy who has no interest in her and so someone who knows less wouldn't understand that and attempt to get them to enjoy themselves. by putting down such a restriction, it adds an extra mechanic to some of the sim population on who they wish to be with, rather than forcing a pair that shouldn't be.
Also the restriction should be an option when children reach their teenage years on age up, if they wish to be exclusive to SGR or the usual way.