Guys lose their balls after they cum.
I've had quite a few guys making kinky requests (such as being peed on, snowballing, being "forced" to eat their cum, etc.), but usually, as soon as they cum, they completely lose interest in acting on those fantasies. The moral of the story is to pee on them BEFORE they cum. I have yet to find a way around "forcing" them to eat their cum after the fact though.