Is it normal that i had a sex dream about one of my female friends?

Before I begin, I'm a guy and in a committed relationship... Ok, here goes...

Last night I had this dream that both aroused and intrigued me. It was involving a female friend that I used to work with, and still have semi-regular contact with (we see each other at parties hosted by my other friend, occasionally at pubs etc.). She is stunningly sexy (though she won't admit that herself) with a nice, petite body, gorgeous face, shoulder length brown hair, smallish breasts, a trim, toned stomach and a bum (ass) to die for.
We've always had a very good, close friendship, and have always been openly flirtatious with each other. And that's where this dream comes in... I can't remember how we got there or the events leading up to it, but for some reason we had gotten back to my house after being out somewhere. Then we went up to my room and she peeled off this hot black evening dress and she was completely naked underneath. I was completely astounded, not to mention incredibly turned on (I've never seen her in the nude in real-life). She, however, was acting like it was no big deal. She could see the lust in my eyes, though, and knew what was on my mind. She slinked over to me and we began to kiss passionately. Afterwards, I threw her on the bed and stripped until I was completely naked and we began to have sex. It was intense, believe me, and I loved every second of it. Soon after we both climaxed I remember us collapsing into each others arms and holding each other tenderly. It was not too long after that, that I woke up.

After stirring and remembering it all, my hard-on grew, but so did my guilt. I mean, I have a wonderful girlfriend (also very sexy, but in a different way- everyone's different after all), and I don't know why I was dreaming about my friend like that.

What I want to know is, is it normal to have sex dreams about your friends, and are there any other people in the same situation as me?