Are you pissed because of a bare butt or because he was hiding something from you? Are you actually feeling hurt, angry or cough threatened by this? Take it for what it is but I think you're making a mountain out of a molehill and he's being inconsiderate and immature. Long and the short of it is that you're both being unreasonable and who is more so is truly irrelevant. Reflect on why you actually feel as you do. Why your partner feels the need to act as they did. Why its an unfinished argument in place of understanding/compromise? This is the one person in which disagreement and tension doesn't have to insight argument... I'm being somewhat sophomoric here but hopefully nudging a bit of perspective.