Why not ask whether heterosexuality exists? Are there truly people out there who are so disgusted by the same sex that they’ve never had a dream, a thought, a moment of desire for another person of the same sex? If so, why do these people have such a revulsion? Is it cultural? Is it biological? Both? We take heterosexuality for granted, but we still don’t know if it truly exists. All primates (with the exception of humans) are bisexual. Bonobos and chimpanzees especially. In human societies that are not corrupted by homophobia or biphobia (see Greeks, Canaanites, and various modern tribes), the people are bisexual. I think heterosexuality should be assumed to be a cultural invention until proven otherwise.