There are actually two very important questions here. If you are going to explain sexual behavior from an evolutionary perspective, it would seem extremely difficult to explain homosexuality. The behaviors of Mother Teresa and Ellen DeGeneres are very difficult to explain from an evolutionary perspective.
Altruism is seen by most people as very desirable.
Until fairly recently, homosexuality was considered a disorder by psychiatric community. Two copies leads to homosexuality. This gene could survive in the population. A gene that makes you very attracted to men. When this gene occurs in women, leads to increased reproductive success.