Some people get very freaked out by sexual fantasies and worry that having fantasies makes them "bad."
When I was in private practice, I did a lot of sex counseling, and one thing I would ask a couple individually is "what are your sexual fantasies?" Here's the typical answers I would get:
1. "Oh, I don't have them. That would be rude."
2. "I can't talk about them. They're really sick."
3. "What difference does it make? He or she would never do them."
There's a lot of angst surrounding sexual fantasies. They're basically erotic thoughts that make you feel sexier. They're arousing and add to the "moment." Research shows most people wish to fantasize, but have sex in the context of the person they love and feel most comfortable with. - in other words, they use fantasy as an aphrodisiac. The people who report the highest sexual satisfaction tend to be those who have fantasies, because they fuel arousal, particularly when things start to get a little "everydayish."
Things are very exciting when they're new, and when they get to be functional, things lose their appeal. That's when people stop playing with each other, stop being each other's girlfriend or boyfriend, and stop using sexual fantasy.
If you're a tennis player, all things you do before a game are like foreplay. You imagine hitting the ball well today, and what you're doing is fantasizing about how the game is going to be. So fantasy provides a very important function. Fantasy takes your mind totally away from distractions and allows you to enjoy the "moment," helping you to focus and maintain arousal. When women say they have trouble with orgasms, the number one thing therapists try to get them to do is fantasize and masturbate. It's like jump-starting the whole mechanism.
In general, the most common fantasies for both men and women are those that relive an exciting sexual experience, or those that imagine sex with a current or different partner. It doesn't mean you don't love your spouse, but it means sometimes you need a jumpstart to make yourself feel sexy and not make sex feel so mundane.
The next most common fantasies involve oral sex, sex in a romantic location, heightened sexual power or irresistibility, and (believe it or not), for most women, forced sex. When women fantasize about forced sex, it's not about being beaten, stabbed, or shot. Instead, it's about being swept off their feet, being held down and totally enjoying the abdication of responsibility for anything happening. Male fantasies tend to focus more on visual imagery and explicit anatomic detail. Women's fantasies tend to be about emotion and affection.
And consider telling your partner your fantasy, but not if it's that your neighbor's body is so hot you can hardly hold your breath, or that you think of him or her while having sex with your spouse. But if you have a fantasy about the two of you, and you know your spouse well enough to know this would not be crossing a line, then you can tell them. Things that won't hurt your spouse's feelings can be shared. Otherwise, keep your mouth shut, because there is nothing UNsexier than telling your spouse "I have to go to an entirely different place to do it with you, but I love you." THAT just doesn't fly.