One of the most important things everyone should know about sex is that it's only one part of a fulfilling, connected relationship—but be aware when chemistry starts to fade. "I think the main thing to keep in mind is sex is a key, key part of a loving, lasting relationship and that when you're having troubles in the bedroom, you don't want to sweep it under the rug. You don't want to ignore it, because it really can take on a life of its own," Dr. Berman says. "The sexual problems create relationship problems, and the relationship problems create sexual problems."

As sex decreases in a relationship, so does any nonsexual physical attraction. "The threshold for anger gets lower. [Couples] fight more," she says. "You become more like roommates or co-parents, if you're lucky."

Take some time to define intimacy in your own relationship—and realize sex is only part of the package. Think about emotional intimacy, Dr. Berman says. "I'm talking about that feeling of connection. Closeness. Trust. Romance," she says. "And women in particular, I mean, that's what makes us tick in a relationship."

Overall, realize that—like any relationship—sex takes work. "It takes effort. It takes connection, and it takes learning what your partner's language of love is," Dr. Berman says. "If you try to love your partner in the way you like to be loved, it may not translate for them. So really learn what makes you feel loved, what makes you feel cared for. And love your partner that way ,and you'll see the changes that happen in your relationship."

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