Im not sure if that could be a big thing to worry about because they could just be friends. But if you think its serious such as her talking to him most her time. Going out with him consistently....
Well, don't entertain her every complaint anymore, that just adds to her paranoia. I would get her some books on older women that have aged gracefully and let her be. If it starts to interfer with...
if your girlfriend has a girlfriend, then she might not fully understand the concept of a faithful relationship, and is clearly Bi-Sexual. just let her know easily how you feel about that, and ask...