An old post back from the dead but great lessons here.
WOW.
ALL women and ALL men have the capacity to cheat and will do so in the right situation and with the right person. The chances of them doing so go based on who she is as a person, but also based on how you are handling yourself and how you are handling her. All relationships involve a value exchange. She should be getting all the male connection, attention, validation, and intimacy she needs from a man, from you. If she needs more, then she either doesn't value you and thus your connection, attention, validation and intimacy don't fulfill her, she cannot be satisfied by one person and is not relationship material, or you are not providing her with the appropriate amount of these things.
First point is that when a women does something like this, it is your responsibility to speak up and draw your boundaries. The trouble is, any woman who does something like this is going to push back and try to have a moral debate with you on whether it is "right or wrong" for her to have a friend she goes out on dates with - and let's be honest here, going out with a guy on a Friday night to dinner and a movie? How much worse does it have to get for him to think it's not a date? Spending the night at his house? Men must get out of the mindset of right and wrong, and must stop arguing with women about it. You draw the boundary and that's it. No discussion about it. She either accepts the boundary or doesn't. If she wants to claim you are insecure, fine, but that's just another boundary she has crossed. She doesn't care about you or your feelings and only cares about filling her own selfish voids.
Second point is men who are so in a woman's frame that they are more concerned with what the woman will think of them than they do with standing up for themselves. I used to worry about the same thing with my ex-wife. She even used the manipulation tactic early-on in our relationship where she complained about her ex-boyfriends being jealous about her male friendships or conversations with other men. Hint hint. The meaning there was, "I just know you aren't going to be like THEM right?" She was literally laying the groundwork to cheat and to get as much male attention as she could. Big red flag.
It is not appropriate for a woman in a relationship to hang out 1:1 with another man. You should be there, or it shouldn't happen. If she doesn't like that, then she isn't relationship material. You must draw this line with women, and you must also adhere to it yourself - no spending 1:1 time with other women alone. If she pushes back on it, you just explain these are the terms of a REAL relationship for you, that you will adhere to the rules equally, and if she isn't ok with it, she should find someone else.