I'd agree with the others in that it means she is probably losing interest. However, I dont think she is cheating on you yet or about to, otherwise she wouldnt be telling you about the other guys advances. It seems to me like she is testing the water about something, trying to see how you react to other guys advances on her. Also, try listening to what she is saying these guys are doin, it may be that she is wanting you to do the sorts of things that these guys are doing. So maybe surprise her with a special treat, and perhaps play out some of the things she is finding so enticing about the other guys.
I would also recommend that you talk to her about it, after all the key to all relationships is communication. Let her know that your feeling a bit threatened, and she should talk you out of it and help show everything is all right.