Sleeping with someone through deception. Which is... eh, I don't know, I can kind of see the argument for it with Mystique (esp since she tends to use sex to manipulate or coerce) but that's a very hazy area especially when it's a one-night stand. Then you run into the problem of "what about plastic surgery etc etc etc".
That said it is sort of a type of violation, mostly because it's Mystique and that's someone who Gambit... probably wouldn't sleep with without the deception. Though I guess most of the point of one-night stands is about getting off and less so about who it's with? Then there's the whole did they or did they not when she got caught impersonating Foxx. I don't know this is a mental headache to work through lol.