I'll answer this on the assumption that someone actually got caught doing it even though the odds of getting caught doing it are really low.
If you were caught cumming in/on something it would classify the same as spitting on someone which can be classified anything from assault 4 at the lowest severity, clear up to assault 1 with sexual intent at the highest severity. If you happen to have any form of disease that could possibly be transmitted in this manner then you're automatically going to the most severe level and getting a charge tacked on for intentionally spreading a communicable disease which is another felony.
