If you two are secure with each other, and you've talked about this situation, then you've just got to let her be. If something happens, you'll find out about it and that will be that. I mean, if it happens wouldn't you rather know than not? And if she cheats, then you two just aren't meant to be. You're not being unreasonable, everybody worries when their significant other goes out drinking, but maybe you should just let things take their course.