M any marriages fail because people base their attitudes and behavior on widely held beliefs that simply aren’t true. One of the most destructive is the belief that if you desire someone else, something must be wrong with your relationship.

Example: You fantasize about a coworker, then worry that you’re “betraying” your spouse. Or you see your spouse eyeing someone and question his/her fidelity.

Human beings have their heads turned by others. It’s just the way it is. It’s not a sign of infidelity.

Of course, there is a big difference between desire and action. Fantasy is harmless and can be better than reality.

Other myths to watch out for...

Myth: Your spouse should be your best friend.

Women in particular often want their husbands to be their best friends. They want them to reveal everything... share their deepest feelings... chat for hours about relationships. Men are rarely like that -- so their wives are disappointed.

Reality: You look for different characteristics in a spouse than you do in a friend. A best friend might think the same way you do and enjoy the same things. But when you evaluate a potential spouse, odds are that you consider sexual attraction... parenting potential... financial solvency, etc.

You can have a great marriage and not be best friends. Couples can have fun and enjoy their time together without seeing the world in exactly the same way.

Myth: You should never go to bed mad.

Simmering anger invariably comes to a boil... and every couple is happier when they “let off steam.” Right? Wrong.

Nothing gets solved when people are enraged. In fact, expressing anger produces even more anger from the surge of adrenaline and other stress hormones.

It’s better to go to bed mad than to say or do something you’ll regret later...

Tell your spouse, “I can’t handle this right now.”

Agree on a time to work out your disagreements.

Once you’re calm, slip praise and compliments into the discussion.

Example: If you are upset about a purchase your partner made, remember to mention that, most of the time, you agree with and respect his money management.

One researcher found that it takes five positive comments to undo the anger-causing impact of one negative remark.

Myth: You should always be 100% honest.

People get angry when I say that it is not always good to be honest with your partner and share the intimate details of your life.

Should a man tell his wife he kissed a neighbor three years ago? Should a woman tell her husband that she has a harmless crush on a coworker?

Don’t use your partner as your confessional. Think twice before you tell something that will forever cause your partner to doubt your word or be jealous of your relationships with friends.

Myth: Sex that is unsatisfying in the beginning can always be fixed.

Great sex won’t necessarily keep a couple together. But bad sex leads to frustration, alienation and/or anger. If sex is unsatisfactory early on, don’t assume things will get better. Some problems -- impotence, painful intercourse and premature ejaculation -- may be solved. Others are unlikely to change...

Differences in desire. If one partner rarely wants sex and the other wants it a lot, the relationship will suffer.

Incompatible sexual tastes. If one partner has “kinky” desires and the other doesn’t, tension is inevitable.

Most partners can learn to satisfy each other, but only if their sexual desires are similar.

Myth: No marriage can survive infidelity.

Few experiences are more painful than discovering your partner has been unfaithful. But an affair is not necessarily a sign that your marriage is over.

There are many reasons why a spouse may see someone else -- the need to feel attractive to others... loneliness when a partner is away... giving in to an opportunity, etc. But these say little about the quality of a marriage or the strength of the unfaithful spouse’s commitment.

If you have been betrayed, ask yourself what is most important to you. If it’s sexual loyalty, then an affair can mean the end of the marriage. But if what you value is your partner and the other good things in your marriage, it’s worth making the effort to work through the crisis.

If you are the offender, knowing there is a chance for forgiveness might mean the difference between choosing to stay or leave. Often, the discovery of an affair provides the impetus to seek counseling to strengthen a marriage.