Regarding marriage, obviously there's a lot of variance, but it's a generalization that at least some people who aren't me make, and I know it's at least true for the Mbuti and the Piraha.

Of course, we'd have to first define the practice first. I'd say that marriage in the broadest sense of the word means that there's some sort of extensive activity (whether legal or ritual) which signifies that people who have romantic or sexual relations of some sort are in some way bonded, which remains in effect until death unless actively nullified.

I bet your average hunter-gatherer wouldn't really know what homosexuality is, let alone be against it, since bands are small and it's a minority phenomenon, but as far as I can tell there's plenty of cultures where it isn't taboo.

Given the diversity of cultures and the difficulty of cleanly delineating modern hunter-gatherers from agriculturists, it's not exactly an open-and-shut case where broad generalizations can be made and the anthropologists doing the reporting are a pretty politically leftist bunch, but I think given the information we have to work with my general impression is reasonable.