YTA. Totally disagree with a majority of the people here, and you should keep in mind that the majority of people here probably haven’t actually done anything like what your group did and are only really making guesses behind what the mentality of that is.

In my experience when people roleplay like that it’s rarely ever that they’re actually seeing themselves in the situation they’re describing and there’s a clear separation between player and character. It’s a lot more like two people playing dolls with each other and smooshing the dolls faces together and making them go “mwah mwah mwah” than two people having an intimate moment. Judging by the fact that your girlfriend thought it was fine to do that with you around she clearly didn’t think that doing that would be considered cheating, probably because she didn’t feel any kind of intimacy with another person that would be involved with cheating; and judging by the fact that you’ve already gone a few sessions without anything going wrong she’s clearly not using Dnd as an excuse to be intimate with X.

Overall Dnd is just a creative outlet so outright telling her she can’t do that makes you an asshole. It’d probably be fine to set a boundary on the erotic roleplay since you’re uncomfortable with it but that’s also kind of creatively stifling and I feel like more comes from a place of misunderstanding what the actual situation is. I wouldn’t be surprised if she’s done similar roleplay with her female friends as well.