If gays cannot reproduce why are there so many of them?

They could reproduce if they were to have sexual intercourse but because of their orientation they will not copulate with the opposite sex. This has no bearing on how many there are. Gay people come from persons of the opposite sex having intercourse, ie, heterosexuals.

This is because homosexuality is part of overall human sexuality. Most people are really bisexual, but heterosexism, still operating even though homophobia is fading, pushes most people to live hetrosexual lives and ignore their homosexual feelings. Most gay people lives are doing so because they are not bisexual enough to give in to heterosexism and lead heterosexual lives.