Some guys have a thing for feet or armpits even though they still have a set of those body parts on their own person. You can't really blame him, because that is what he's attracted to. The main point is that even if a gay guy could already produce his own semen just by masturbating by himself, he would still seek the taste of someone else's whenever he gets the chance to do so. Cum, semen, jizz, spunk, seed, spurt, load, skeet, whatever you want to call it. Every gay man's dream is to be able to taste the nectar that comes from another man's penis.
