What is it?

So my fiance and I have been going through it. He ended up getting covid but now he's better. But we haven't had sex because we were arguing a lot then when we got better I got on my period and he got covid. So he was teasing me all day and he even took a shower and made a comment as if we were having sex tonight. So then when we were laying down. He fingered me and got aroused and I started touching him. Then he started talking about our son because I hit the remote and he then said he didn't want to be intimate anymore and he said he just wanted to cuddle and talk about our son. I don't mind cuddling but you made it seem like we were going to be intimate and it's almost like a joke to him at first I thought he was joking but then he said I'm not in the mood. So anyways before thats. I noticed he has some white stains . Almost looked shiny but not too noticeable on the outside of his boxers( I wash all the clothes) so then after he didn't want to have sex I just got curious and checked another pair and this time it was on the inside of his boxers but the stains looked more obvious . I've seen them when we had sex. What could this be ? I don't want to assume he's cheating but is he masturbating? I've never seen these stains. And they're so discrete.