I think it is natural that most are most strongly attracted to the opposite sex as natural selection would make it that way, but I don't think natural selection would make us sexually exclusive towards one sex only. I think that is a socially created behaviour that we've created.
Sex is not necessarily only for reproduction. It could strengthen social relations, relieve stress and, frankly, it feels good. Bonobos have sex with eachother all the time and it is believed that it has evolved as a social lubricant; to avoid conflicts, strengthen social bonds and promote peaceful coexistence. As a result, bonobo chimps are much less violent than "regular" chimpanzees. Human sexual behaviours have evolved for reproduction (of course), but could also have evolved in part for other benefits, like those the bonobos enjoy. And of course, two sexual partners don't have to be of opposite sex to enjoy such benefits.
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