She's afraid you'll recruit her bf into the Gay Agenda™.
Joking aside, this is usually the underlying case when a bro's girlfriend gets "concerned" with him hanging out with gay dudes.
The only really understandable reason for her to cock block your bro time is when her man starts picking up "gay habits" from you or your lgbt friends. Like he's still totally straight, but he's doing things the average woman wouldn't associate with a heterosexual man to the point he seems a bit metro. This can invoke a straight woman's concerns. This is part of how I know that a lot of modern women still have a lady boner for a man adhering to a classic masculine gender role. They don't find him as attractive otherwise.
If that wasn't the case, they would be no concern over a woman's boyfriend or husband acting effeminate.
Concerning that I don't act like a stereotype and that my voice is deep, she has no concerned. She often said I looked straight. ---
Losing an argument? Ends it with but...but...Hillary and her emails!