NC because TMI (sorry) and here because I didn't know what topic this falls into.
BF and I haven't long been living together, all going swimmingly but obvs still getting to know each other etc. This morning he was out & I went to change the bedding, finding what I can only describe as a couple of skidmarks on his side of our white sheets. 

Obvs I just whacked them straight on a hot wash & carried on remaking the bed but now I don't know what to do. I don't want to bring it up and embarrass him (I'd be mortified if it were the other way round) but neither do I relish the idea of it happening again. He always sleeps naked unless he has a 'funny tummy' which now makes me think he might have some experience in this field.
Please sensible ladies of MN, what's my best course of action? 