I agree with DavidSoutham and Dradebaugh. I know I wouldnt take the risk. Could be in danger of catching STDs. Of curse, if you talk about it with her, she will say she was fantasising, but why would she fantasise with someone? In her mind, she s already cheating on you. How long before it comes to reality? You should find a good person. You will find that person when that person doesnt get steamy and intimate before marriage.
I have never heard of that in an everyday normal relationship. But I don't know...just ask her. She HAS to know she's doing it. It seems as if she was having an affair she would be extra careful NOT to yell out the guys name...just ask her. Maybe it was a previous lover , I mean " luvuh ".