While I see where you coming from, I'm not sure it would be a beneficial addition to the base game.
It would drain design time for a feature, that won't be used by many. (Most people don't bother to change the default settings in many applications, and this includes CK3 as well, where in default you have a slight minority of non-straight characters.) The problem comes with inheritance, which is very common in this game. If a gay couple has a child whose dynasty should said child have, which claims should they inherit? Unlike in most design choices you don't really have any historic guides to go by.
Another question is how to aquire the child in the first place? Adoption, concubinage, kidnap are available solutions, but they all have problems.
I would rather see dozens of unofficial mods coming up with different solutions, than an official one. It's also better for modders, who came up with some unique idea to compete with one another instead of competing with PDX's solution.