By the way, Farinata, I think I remember reading somewhere that the "natural" (= naturally selected for) state for humans and chimps is bisexuality. This allows for the advantages of heterosexuality (procreaton) and also those of homosexuality (male-male bonding that reduces social conflict.) You can't make war with those whom you screw.
Therefore, Shaun's point that if homosexuality was universal society would not procreate is not quite true. It's true only if everyone was EXCLUSIVELY homosexual, but in fact many people are both (bisexual). Obviously homosexual sex has the disadvantage of spreading STDs more easily, heterosexual sex has the disadvantage of unwanted pregnancies. You pays your money and you takes your choice. Speaking as a heterosexual.