Gay couples can still have children. It may not be both of their genes producing them, but that's still another couple raising a child that would have not otherwise been there. Just because the baby wasn't completely birthed by them, doesn't mean anything.
It's no different than a heterosexual couple who do in vitro fertilization as well.
A same gender couple can adopt or do in vitro. An opposite gender couple can adopt, go the in vitro route or conceive through intercourse.
The same gender couple however, never has to worry about the unplanned while the opposite gender couple does.
Not making lite of your post (although I have a feeling some will think I am), but that still looks like the relationships are quite different.
Peace cowman