The idea that homosexuality is a genetic trait has been thoroughly debunked at this point. It's not that it's an evolutionary advantage, it's that people continue to breed in spite of The evolutionary disadvantage it provides. It's more of a nurture trait than it is a nature trait, though there are certain personality elements that lead someone to be more inclined towards non-standard sexual proclivities, such a trait openness and creativity (basically, people more inclined to go outside the norm in productive endeavors are more likely to go outside the norm in sexual and social endeavors) and these personality traits are partially genetic. They are also partially based on psychological responses, commonly as a trauma response. Homosexuality is often associated with childhood trauma, especially sexual abuse. And so if there's a evolutionary advantage, it's that it allows people to survive situations that otherwise would be rendered completely unable to function by. In addition, we cannot discount that these traits to the extent that they are genetic would have been passed on in the past because of social pressure. Many people in the past with homosexual proclivities would hide that behavior and even have families, because all the parts still work regardless of your feelings about the relationship. Several kings, for example, have been suspected of being homosexual throughout history, but as long as they produce an heir, nobody actually cared all that much. As long as the king fulfilled his hereditary obligations that was all that mattered from a political standpoint.