He’s not up for sex.

That’s right, he has quit putting out. Since “Honey, I have a headache” is often considered a female problem, it may be bewildering when it's the man who says it. Whether sudden or gradual, his zero sex drive can send her mind into overdrive.

What’s wrong with him? What’s wrong with me? Why doesn’t he want sex?

There are an estimated 20 million Americans in unions where the male has quit being sexually intimate, leaving a number of women are asking these questions.

In female fashion, she’s analyzing the issue to death. Why is he so annoyed or exhausted by the prospect of sex? Is it physical, emotional or psychological? Is it all of the above? Or has something gone awry in the relationship?

According to the authors of “He’s Just Not Up for It Anymore,” the following are among the many explanations women come up with when sorting through this sorry state:

1. He’s angry with her. She’s not sure why, but withholding sex is one way of getting back at her for something she must have done or said.

2. He must have been traumatized at some point. Some sort of past sexual abuse or violence has impacted his ability to sustain a relationship or get in touch with his sexual core.

3. He’s not well. This could be that he’s depressed or is limited by a physical ailment. In either case, his health is to blame.

4. He’s an addict. She begins to suspect that he’s a closet alcoholic or dependent on drugs.

5. He no longer finds her attractive. While she knows it’s unfair, she’s all too aware that her weight gain may be a turn-off. This includes feeling hypersensitive to the fact that he may find her post-baby body unappealing.

6. He has a problem with porn. He’s probably looking at porn every chance he gets. She rationalizes that she can’t compete with his preference for porn. She can’t live up to the unrealistic images on his computer screen. She’s being replaced by images that have desensitized him to her.

7. He’s cheating on her. If he’s not getting it from her, he must be getting sex somewhere else.

8. He never wanted sex in the first place. A sad reality for some in this situation is that intimacy ends before their first anniversary. She’s not sure if this is a matter of boredom, dissatisfaction or the slight possibility that he’s gay.

Naturally, the possibility or reality of any of these “justifications” can drive a woman crazy. This is especially true in situations where she enjoys sex or she’s making attempts to be adventurous in the bedroom. His lack of reaction can be even more confusing and painful if she knows that he still masturbates. This indicates that the problem for him isn’t physiological, but more situational.

Ultimately, she feels lonely, hurt and abandoned by his seemingly illogical behavior. All of this takes a toll on her self-esteem and ego.

So what’s a gal to do?

— Create an emotionally safe environment for him to deal with this issue, as well as your day-to-day affairs. Don’t be critical of him, nagging him about the situation. Instead, share your feelings. Ask him to help you understand what’s going on.

— Consider what’s causing him anxiety in the bedroom. In some situations, you can blame Ron Jeremy (and other well-endowed porn stars) that have left some males self-conscious about their size. The imagery he’s seeing may be causing him performance anxiety. Alternatively, he may be suffering from erectile dysfunction and fear "failing" in the bedroom, hence he is avoiding intimacy.

— Ask him to be physically examined by his doctor to rule out any reasons for low or no libido.

— Suggest counseling. While he may initially refuse to seek help or take the steps to get to a better place, he may eventually warm up to tackling issues, including in therapy. (You can look up a sex therapist in the Society for Sex Therapy & Research directory.)

— Pursue a healthier lifestyle. Making adjustments like increasing your exercise routine, eating a more nutritious diet, or having fun with leisure activities can help remedy some physical and mental reasons for a lack of libido. Taking care of each other through such activities can help bring back the spark — and make both of you feel more attractive for the other and for yourself.

Dr. Yvonne K. 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."

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