fantasies in my head

Hi, I hope no one reading this finds this to be distasteful to read, but I could use an anonymous opinion.  I'm a 31 year old guy...average white suburban middle class Joe, no history of extreme weirdness in my life.  My question is why do I have the intense desire for my girlfriend to have sex with other men?   I'm not bi or gay I assure you, well I might be .05% bi since I've vaguely "thought" about it a few times, but thats about it.  Something about the scenario of her cheating on me, or somehow being very promiscuious turns me on alot.  In some circles, this scenario is known as "cuckolding".  She knows about my thoughts and doesn't have any problem whatsoever with it, but she won't outright do anything..."maybe some day who knows" as she puts it.  The relationship we have is almost eerily idealistic in every way...we are a perfect match.   So, do you think something is "wrong" with me?
Thanks for listening.

[Roger Gould, M.D.]

These kind of erotic fantasies are very common, and quite normal..there are many explanations for particular fantasies, but too complex to go into here.