Sirius didn't really try to block what was coming, instead sort of rocking back and catching his balance again. Having no issues knowing any details about James and Evans - which was not to say Sirius was dying to know or anything - Sirius just didn't see what the big deal was. He just wanted to talk about it with someone, and since it was about Remus, and Peter was even more squeamish than James, well.
"Yea, it's funny," Sirius shrugged, "but sometimes I just want to talk about it with someone without it feeling like it repulses them." He wasn't really looking at James. He hadn't thought about what he was saying as he said it, but he hadn't meant to be so frank about it.