In evolutionary terms, it we weren't biologically designed to be heterosexual, there'd be a lot less of us around now. I'm not denying the homosexual aspect of many species, but every animal is biologically designed to mate with the opposite sex. Thus, I'd say heterosexuality is down to nature, but human attitudes about sexuality are down to nuture.
we were designed to be heterosexual otherwise we can't reproduce.
I hate to say this and I'm sorry to any gay people I offend (if I do), but homosexuality is against nature in the way that two people of the same gender cannot reproduce. Heterosexuality is needed for reproduction and hence the evolution of the species.
Of course it is natural, we need it to exsit, unlike homosexuality.