Most Helpful Guy
It was either pee, precum, or semen. Usually you can't see that it is white on clothing so it couldve been any of the three. However he would've changed if it was pee because it would he everywhere, so we can eliminate that one. If it was precum, he was probably just fantasizing and got turned on. If it was semen, he either masturbated or had a wet dream. Either way, I'm sure he wasn't cheating on you. Think about it, if he was cheating on you, his semen would be inside her, not on his boxers... But if he is sexually deprived enough that he bad to masturbate or had a wet dream, you need to boost the sex life between you two, otherwise he might end up cheating which is what you are afraid of rn.
Most Helpful Girl
You're definitely over reacting.
As long as he has a pulse... you're going to see that a lot.