^ That.
Not all tribes were paradises of acceptance and equality, only later tainted by Evil White People. Some had just as strict rules as the Middle Ages about status, sex, and sexuality and were just as brutal in their enforcement of their rules. It may only seem like some tribes accepted (or didn't care about) homosexuality because they funneled most of their energy and effort into survival alone, and worrying about who prefers to get squelchy with whom is a comparative luxury.