Answer to actual question of thread: If I were to suddenly become homosexual and end up with a good friend's Shwoop-De-Whoop-A-Golly-Golly in my mouth, I would swallow. I feel like there is something about spitting that kills the intimacy aspect of the act. It would be like someone going down on a woman but saran-wrapping them first (which, admittedly, is a real form of STD protection called a "dental dam," but in my opinion there is no reason in the world to be sampling the local cuisine with someone who you suspect may have a burning sensation they haven't clued you in on).
Answer to the cover up question: Pond and lake water is full of mostly harmless bacteria. Unless you're drinking from the Tennessee River, you'll probably just build up antibodies.