It happens to even the most passionate of couples. At some point in the relationship, one lover’s sex drive is in seeming overdrive while the other one can’t even crank up the engine.

The occasional "not tonight" goes from being no big deal to a major one, as rejection becomes a regular routine. So what’s going on? These libidos are mismatched.

Click here for Yvonne's latest video blog: Why Isn't Body Hair Cool?

Ask any sex therapist and you'll hear, hands down, that mismatched libido is one of the most common problems seen in practice. One partner is enthusiastic about being sexually intimate while the other is subdued. And this takes a toll on the relationship.

The damage: Either lover can feel hurt and frustrated by his or her partner’s attitude or sexual desire, i.e., what the lover wants to do or refuses to do. They can feel disappointed if they’re not into the moment or act. They may feel sore if their needs aren’t being met.

Throwing the couple for an even bigger loop is that they’re still in love with each other — and love isn’t conquering all. They’re also frustrated that they’re not copulating like the Joneses.

In processing mismatched libido, couples have to realize three things:

First, our society promotes sameness when it comes to the bedroom. There is supposedly one way to have great sex. This involves lasting a long time, climaxing with ease and staying hot-and-bothered forever. If you don’t fit into this stereotype, then you’re labeled inadequate, dysfunctional, abnormal, deviant or inhibited — even if there isn’t a "problem."

So if it’s your lover who isn’t reacting like everyone else, you can wind up feeling hurt and confused. You come to doubt your sexuality. You start to question your love for each other.

Second, expectations are at the heart of the mismatched issue. Yes, we can blame Hollywood, erotica, romance novels ... and the sexual role models they provide. These people never struggle in the bedroom. Yet what’s proving to be a real issue is that our expectations have become complex and detailed in recent decades.

Lovers have a lot more information — and misinformation — as far as one’s sexuality goes. People expect pleasure during lovemaking and for one’s sexual response to come easily at that. And when they don’t react as expected, then there’s something "wrong" with them. This leads to tension.

Third, both partners play a role in being mismatched. More often than not, the partner who is unresponsive or uninterested is blamed for the situation. What couples need to realize is that just as it takes two to tango, it takes two to tussle. This can be in:

— How sex is initiated and differences in raising interest;

— How a lover reacts to the other’s desires, especially in not being understanding or supportive; and

— The inability to deal with disappointment.

Whether a partner "gives in" to a request or withdraws, either partner may end up sulking, criticizing or feeling agitated. Further, negative reactions can lead to hostility, strain, anger, thoughtlessness and even cruelty. All of these reactions are unproductive and are the reasons the couple ends up in therapy.

So knowing all of this, how do you move forward? Couples need to realize that every one of our sexual abilities, expressions and preferences is unique. Sexual styles are unique.

Couples need to embrace that there are multiple definitions of what constitutes "adequate" or "normal" in the boudoir.

In actually dealing with the situation, couples need to work toward positive, effective communication. Couples often can’t talk about their mismatch issues without becoming upset. Things end up in a stalemate despite their efforts to support one another.

So what’s the remedy?

Become knowledgeable about the behaviors or dysfunction at hand. Handle things calmly and with confidence. There’s no need to apologize, feel guilty or be submissive for your libido or desires.

Really listen — recognizing and validating your partner. Are you "present" in processing what’s being said? Are you being reactive? Talk about the importance of sex, your hopes for the sexual relationship (e.g., amount and length of time, the need for affection, who should take the lead, variety, etc.) Describe your mismatch objectively and without judgment.

Pretend that you’re analyzing another couple, if that makes it easier. Discuss the role of sex in your union and the effects of the mismatch. Then figure out how you can reverse its effects with solutions.

For example, if one lover is a morning person while the other is a night owl, this incompatibility in energy cycles can impact the amount of sex. This couple will need to plan on a compromised time frame for sexual intimacy — and conserve their energy for such.

Finally, do a "strengths analysis" of your relationship. After all, it’s important to remember where you’re matched and what is actually working.

In the Know, Latest Sex News …

Have that sex talk — again and again and again ... The American Academy of Pediatrics is urging parents to go above and beyond a one-time "big talk" with their children about sex. In their study, as has been previously reported, researchers found that teens who talk repeatedly about sexuality matters with their parents feel closer to their parents. They also view their sex discussions as more open.

Protein could lead to HIV vaccine. According to the Globe and Mail, a protein found in the nucleus of CD4+ T cells, known as FOX03a, may be the key to an HIV vaccine and new treatments. Researchers at the University of Montreal, McGill University and BD BioSciences believe their findings also may help in developing therapies for other diseases, such as hepatitis C and some cancers.

Sex on the brain at University of Iowa. Upon hearing that the university offers more than 35 courses with the word "sex" in the title, Iowa Republican state Rep. Mike May became critical of the school’s offerings. He feels there are other courses that would be more worthwhile given today’s job demands.

While, at quick glance, classes such as "Gender and Sexuality in French Cinema" may seem like a waste of time, many at U of I are pointing out that such courses delve into an issue’s psychological, philosophical and ethical considerations. All of these equip students with the information they need in certain fields.

Click here to read more FOXSexpert columns.

Dr. Yvonne Kristín Fulbright is a sex educator, relationship expert, columnist and founder of Sexuality Source Inc . She is the author of several books including, "Touch Me There! A Hands-On Guide to Your Orgasmic Hot Spots."