I don't know how you ever know for sure if someone knows your trans before it gets to the hot and heavy stage with lots of petting. But if you're post-op and are just in it for a one-night stand, no need to tell. However, if you want a relationship, you should obviously tell beforehand and not let it get that far. Just like a cis girl would. A lot of it will depend on the quality of the vaginoplasty as well. I don't think sweeter words were ever spoken: vaginoplasty!
If you're pre-op and it gets that far and you're drunk and you need it to stop, one word: period. It'll stop. A guy still might want you to, ya know, satisfy him. With women, you don't have to worry about violence. (though it is still a possibility as most women can beat me up.) I also think they will be more understanding if it gets that far. But not always. Just in general.