Everyone has sexual fantasies from time to time, and they're usually nothing to be worried about. Sexual fantasies are not only common, but can be good for a couple, too.

"It's almost always healthy to fantasize," says Laurie Puhn, family and couples mediator and author of Fight Less, Love More. "It only becomes a problem if you are always fantasizing about the same person, and it's a person you have access to."

In other words, if a fantasy of having sex with your spouse's very best friend plays constantly in your head, that's not healthy and can spell relationship trouble. But if you imagine yourself having sex with a complete stranger, or with your partner in an elevator in the building where you work, these thoughts can be erotic and titillating in a good way.

"Fantasies are imagined sexual behaviors that can be shared with your partner," says Terri Orbuch, Ph.D. "Some couples like incorporating sexual fantasies into their relationship, which is fine." But, she says, it's crucial that couples communicate with each other and listen to one another's concerns if they decide to tell each other about their individual fantasies. And, Orbuch adds, "Discussing sexual fantasies is separate and different from engaging in them."

Men and women tend to entertain different sexual fantasies. Here are some common sexual fantasies for men and for women.

The Most Common Sexual Fantasies for Men

1. Having sex where they shouldn't, like in an elevator, your office couch, or at a public beach. "The idea is to have sex somewhere where it is forbidden," Orbuch says.

2. Having sex with a woman who's wearing candy underpants or who's dressed in sexy silk undergarments, Orbuch says.

3. Having a three-way sexual experience in which a third person becomes an active participant.

4. Coming home and finding their partner dressed in nothing but a fur coat, then removing the fur coat to have sex, says Lauren Mackler, relationship expert and the author of "Solemate." "Men tend to fantasize about being seduced and having a strip tease," she says. "There's the one where a secretary whips off her glasses, lets her hair down, unbuttons her blouse and has a sex kitten outfit on underneath."

The Most Common Sexual Fantasies for Women

1. Having sex with someone they don't know. "This fantasy's not about sexual coercion or violence, but simply having sex with someone with whom you are not in an emotionally oriented relationship," Orbuch says.

2. Being intimate with someone as you incorporate sex toys or vibrators into the experience, Orbuch says.

3. Having sex with a man who wants to massage her with lubricants and oils in all the right places, Orbuch says.

4. Having sex with someone while her partner looks on, Mackler says. "This is a common fantasy for a woman," she says. "And it can be perfectly healthy, as long as it remains a fantasy and makes a couple's sex life better."