I think you have answered your own question. The reason many human beings are homosexual is that we are 'higher' animals, just as human beings have so many other unique psychological and social capacities. It also involves aesthetic appeal, interpersonal 'chemisty' and our ability to communicate in mutually fulfilling terms. And consider how much richer the world is for the presence of lesbians and gay men. Your question is based on a false premise.
Some animals do engage in homosexual activity.
I think the existence of homosexuality in nature is, for the most part, irrelevant to contemporary debates over civic equality and social acceptance for lesbians and gays. Homosexuality occurs in nature, and even if it didn't, we human beings do plenty of 'unnatural' things to which no one objects, which we even consider laudable. Animals don't write symphonies, build hospitals, fly airplanes or wear clothes. It's equally silly to claim that because homosexuality occurs in nature that we ought to accept it. Murder, rape and cannibalism also occur in nature.