Dry cum??

So when you and your love one have sex and you do it without a condom and he pulls out before cumming inside then whips his tip off, cuddles and starts to notice dry stuff on his dick and he asks "what is it?" 
I think it's just my dried cum or personal lube... But I still don't know. 
He said that his friends at work told him that, that only happens if the girl been sleeping with other guys and had came into her. 
1) I love my husband and will do anything for him
2) I've never cheated on him during our relationship/ marriage
3) I'm pretty sure his friend is talking out of his ass because ya know what his friends know more about parts cars then lady parts and how they works.