I'm not in a relationship or anything right now but I ask this because I find it an interesting point of discussion and there seems to be a bit of contention on how to handle this.
First off I strongly believe that male-female platonic relationships are almost impossible. There can be some exceptions: a girl can be friends with a gay guy, they can friends by association (guys can be friends with their buddies wife), but a truly platonic relationship between a man and women, where neither of them are attracted to one another, is extremely rare, albeit I guess it is possible in theory.
Anyway, if you're seeing a girl, and she hangs out with guys friends (especially if its 1 on 1) what is your reaction? Basically I see a few different options
A) You act aloof and indifferent and continue your own life and developing your own hobbies, skills, friendships, etc
B) You set boundaries and tell her you won't tolerate it (or break up with her if she doesn't)
C) You only date women who only have female friends
I personally prefer to go with C...but lets say you screen for that type of girl but later she casually mention she hung out with a guy (even if she fabricated the whole story as a **** test) how would you react in that moment? I'd honestly probably just be completely indifferent and withdraw attention or even stop seeing her completely it was negatively impacting our relationship.