I (M23) jacked off to my girlfriends (F23) underwear and she was furious. Seriously, what did I do wrong?

I (M23) was doing laundry with my girlfriend (F23). I had to go to the bathroom so I left the laundry room. But when I came back I saw her dirty underwear on the ground. I picked it up and was going to put it in the washer but something strange came over me. And, well, I jacked off in her underwear. I put it in the washer and forgot about it. Well, she found the cum later and told me to never do something like that again. I don’t really get why she’s mad. I mean, it doesn’t hurt her. I just wonder, what did I do wrong?