Know what, fear of STDs is all good because the threat is real. But syph and the like are pretty treatable. And true, HIV is scary and you'd have no way of knowing if someone is HIV-positive or not... but again, it's just a kiss. Nobody's asking you to drink their blood or something. Lighten up about it already dude.
Treat it like Approach Anxiety. The fear will be always there but the only way to overcome it is to approach anyway. So kiss anyway. I swear it rocks even if you barely know the girl and she's making all these wonderful noises. Haha.