Well, if two gay people want to get married in order to pool their resources so that they can afford to adopt and care for a child who otherwise would have been without a family, doesn't that fit your definition? That is what a lot of them are after: a partner to help them care for children, even if those children are not born to the couple as a couple.

Well, perhaps that doesn't fit your definition, since you say legitimate succession...but then again, you can pass beliefs and traditions on to your children even if they aren't your children by blood, and those beliefs and traditions (whether cultural or religious) matter a lot more than the bloodline.