One day last week, I arrived home a bit earlier than I usually would after work. When I got in, I could hear noises coming from upstairs. There was a sound coming from the bedroom. It sounded like the bed creaking, like someone was bouncing up and down on it. After I walked upstairs (while wondering wtf was going on), I walked into the bedroom and saw what was happening. He was wanking and simultaneously sniffing my knickers. I was standing there for about 15 seconds before he even noticed I was there. It was like he was in a trance. This was the first time I've caught him doing it, but I realise he's probably done it before multiple times without me knowing.
He has said he doesn't think it's a big deal, but he won't do it again because it's upset me. The thing is if he had just asked me about it before, I probably wouldn't have minded. It's him doing it surreptitiously that's made me feel weird. What are people's thoughts? Am I right to be ticked off about it, or am I just overreacting?