Sometimes the husband is more sexually adventurous or becomes so later in marriage. The wife is less interested or does not learn how to make love more enthusiastically. People have sexual cycles. As one spouse's energies begin declining, the other may look elsewhere. Marital problems are also linked with aging. The hormonal changes women begin experiencing as they approach menopause are obvious examples.
The genuine "love" and "affection" can also decline or disappear from a marriage. We are fallen beings and our love lives are not unaffected by this. Generally speaking, sex in a genuine loving relationship is better than without that caring.
There is also the "ball buster" syndrome of wives who henpeck their husbands. That FOX series "Married with Children" exhibits a classic, if campy and farcical, variation of this. Never do this. I have seen women who trash their husbands verbally or argue with them senselessly about every nutty topic. "Denial of affection" takes many forms and it is one of the logical legal grounds people find for divorcing. Tragic, but true. If they don't have a firm commitment to family values and raising children in a loving environment, all of these imperfections and inadequacies can take over and destroy the marriage. Particularly if either spouse is not grounded in a sound moral system and or is not faithfully dedicated to their spouse in a spirit of charity and mutual respect. If love, respect, and morality begin disappearing from either spouse's soul, you are going to have BIG problems.
Whether a husband "cheats" will also depend on his own moral character and what kinds of sexual attitudes he learned from his father, peers, and from the secular culture. It can also depend on what kinds of femininity and styles of womanhood, motherhood, etc., he has been exposed to. There are countries in Europe, for instance, where greater elegance in femininity is more regularly apparent than in America with our growing "Hillary Syndrome" and butch careerism among females. American television, popular culture, and the feminist movement have done a wicked job of stripping womanhood of many of its charms and mysteries. Women, like men, are subjected to systematic moronization by the unending waves of secular humanist propaganda and disinformation.
And, of course, younger women can seem more appealing as one's wife ages depending on her health. Both spouses have to accept mortality and the aging process, realizing that sex is after all finite.
There are also some rather familiar reasons which lead to adultery which ought to be directly obvious.
