Particularly if it has not been agreed in advance.
This happened to me recently when I slept with someone new. I told him to stop and I told him not to do it again. I do not find it remotely sexy. He also spanked me which I was particularly annoyed about because I had asked him in advance not to do this, due to the fact that I had a very bad experience with BDSM in a relationship which was very abusive and damaging to me and which took counselling to sort out. I had flashbacks and everything before it was resolved! He knew all this and still he did it and I told him never to try it again.
I was pleased that since my counselling I was able to be assertive about stuff I don't like. It shows my boundaries are fixed to some extent at least.
But why would anyone think it was ok just to spring this on someone? To me it suggests possible deep rooted dislike of women. AIBU?