It's normal. Within reason, that is.
We don't stop being human when we enter into serious, committed relationships. Of course you will still be attracted to other people--that's a human reaction to people we find pleasing to the eye. It's not wrong or awful--it just is. And yes, you will have fantasies about some of these people, whether it's a co-worker or a movie star. Opinions vary as to whether or not such fantasies are detrimental to your committed relationship, but it certainly is normal to have them.
The problem, like so many other things in life, occurs when we indulge these thoughts--for example, letting the fantasy overpower our real-life relationship. If you want to rub one out every now and again while thinking about Ryan Reynolds, fine, but if you can't ever have sex with your boyfriend without imagining that he's Ryan or your hunky co-worker in Marketing, that's a problem. For starters, it could be a sign that your boyfriend is not meeting your sexual needs, and that's something you will want to address before you marry him.
The other way we indulge them, of course, is to act upon them, or even to be willing to act upon them. We know in our minds and hearts when there's more to a fantasy than the fantasy. If you have sexual thoughts about someone you know in real life and you start thinking about how you can make them happen, you're already in trouble. You haven't cheated--yet--but you are headed down a very dangerous path.
You just have to know yourself and be acutely aware of your intentions. We can't stop ourselves from thinking of something, but we can prevent those thoughts from leading to action that will destroy a relationship we value. A sexual fantasy is like a snake (if you will pardon the pun): you can take it out of the cage and play with it sometimes, but if handle it too much, sooner or later the snake is going to bite you.
If you have any doubt as to whether you can remain faithful to your guy, you had better put the marriage plans on hold and finish sowing your wild oats. Otherwise, what you're feeling is normal and harmless. Just make sure it stays that way.
Thanks for the question.
I love WA's answer; but I would like to add my answer to her first question: Yes, it is possible.
I have met a guy who has been my sole source for fantasies for the last six years. The first two years, I still thought about other guys, ex-boyfriends, the guy behind the deli case at the supermarket, etc. But after a while, I'd see a hot guy, appreciate the eye-candy, and the thought of being under the random dude would suddenly make him not hot anymore. Now I just stop at the appreciation stage.
My boyfriend still fantasizes about other women; I'm cool with that. His healthy libido is one of the things I love about him. I know he isn't going to act on his fantasies. It's not in his nature; he would break up with me first.
So, yes, it's possible; improbable, but definitely possible.
Great answer by Cary.
Happily married people are still capable of becoming attracted to someone not their spouse. It happens even if you are not looking for it. When the someone else is attracted to you as well is when it can become a problem. Never act on it but more importantly do not ever let it get to that point...
It's alright to fantasize to a certain extent. If you act on it, that usually leads to cheating and heart ache. There's always sex toys, adult movies, books etc to rev your engine.