Girls, Is it ok for a girl to have two boyfriends if it actually benefits everyone involved?

My girlfriend and I have been together for five years. She is absolutely amazing and we have a wonderful relationship. She is very beautiful and I have become use to guys hitting on her and trying to get her number when she is at work or elsewhere with a couple of friends. I am not controlling or the overly jealous type and she respects and appreciates that. She is very straightforward and honest with me about everything and dosent like to hide anything. So a new co-worker of hers has been abnormally persistent at this. I have always known that she is physically attracted to black guys, so I naturally paid more attention to this situation. I asked her if she would like me to say something to him, and she said no she will try and take care of it herself like she usually does. A week later she tells me he has not given up at all. She said, I just do not understand why he likes me so much when he knows I have a serious boyfriend that I would never want to leave, and especially after being shot down a thousand times. I actually think he wants me even more now because of the amazing relationship I have had with you for five years. I asked her if giving in had ever crossed her mind if this continues to be an issue? She said that all she knows is it's really becoming a lot more stressful because he is so nice to her about everything. I said, I know you have a "thing" for black guys, are you attracted to him? She said yes, I am actually. I then asked, what do you think this guy wants from you? She said, I know that if I give him my number he will want to date, and definitely start having a lot of with me! I said, it sounds like you are mainly interested in that second part! She smiled and said, is it that obvious? He is really y babe, do you think you could possibly be ok with it if I let myself give in this time? And, does all of this turn you on now that you know I want to let this happen? I said yes it really does turn me on and you know I love you more than anything no matter what, and if it happens, you also know I will stay faithful to you and respect and support your decision. She said if he asks me for my number again, which I know he will, I will be more than happy to let him have it!