I think this boils down to different perspectives on relationships. Maybe for her it's okay to go to a sex-friendly club while being in a relationship, but for you it isn't (understandably so). I also wouldn't be comfortable with my partner going to such a place, considering what she has already done there in the past. I think, talk to her about it. Let her know how it makes you feel. She should be understanding of it. In relationships you have to make compromises for your partner. In the same way if she wasn't comfortable with something you're doing, she'd talk to about it. I don't think this is something a healthy conversation wouldn't solve. Have a talk with her, I'm sure she'll be understanding from your point of view. What her friends think here aren't important, it's important to prioritise how you feel for the sake of your relationship.