I've adopted a few times in the game and I generally like how it works. Same sex couples tend to adopt in my game such as the Heckings. I agree that I generally wouldn't think of them as potential spouses. I don't know what will happen with Duane Talla because the way I've played him, he was only briefly fostered as a teen by the Kealohas through a community tribal approach so he isn't a typical adopted kid. Even then, I can't say that he'll end up with Nani, the age gap might be too wide.
I should incorporate adoption more often. I agree that it's interesting to see how the kids turn out as they grow up.
