In an ideal world and in an ideal relationship, love is eternal and the two people involved have eyes for no other. In the real world, sustaining a loving relationship is hard work and the world is filled with lures and temptations at every turn. There is never valid justification for either partner to turn to someone else and abandon the emotional bond that holds their marriage together. But it happens none the less.
Adultery has long been thought the province of the man in the relationship. Stronger sex drives, more opportunities to stray or simply the chauvinistic belief that men just can't help themselves have all been used as excuses to explain affairs in men. A woman is often castigated to the role of harlot or fallen woman if she strays from her marriage bond.
No one can deny that the role of women has changed radically over the last few generations. Women are more often in the work force now, they have their own money and they understand their options better than ever before. As such, they no longer have to tolerate marriages that do not fulfill their needs, whether that need be physical or emotional. Women occupy a greater place in the world and are, therefore, much more likely to encounter interesting and desirable men. Whether it's planned or happens simply by accident, women can, and do, fall in love outside of marriage.
It's hard to find a reasonable explanation to offer the betrayed husband. There is so much more to the bond of marriage than simple physical urges or even the emotions that are supposed to exist in the marriage bond. The decision to marry carries with it a level of commitment that requires both partners to forsake all others. Falling in love outside of marriage is usually a fulfillment of something physical or emotional that is currently lacking in the marriage.
Even with the need fulfilled outside the marriage, the affair cannot carry with it the level of commitment needed for a love bond. No one can commit to someone else until they have released themselves from their feelings of commitment to all others. If a woman has truly fallen in love outside of her marriage, she has given up on the existing marriage itself. It is no longer simply a sexual affair; it is not seeking attention and support that is lacking in her marriage. To fall in love outside of marriage is the ultimate red flag that the marriage itself has ended. The emotions of a dead marriage are in place long before either partner walks out the door.
Extramarital affairs and falling in love outside of marriage are two entirely different situations. An affair may be sexual; it may be emotional. It is never a commitment of love. There may be a resolution for a woman having an affair within her marriage. When the new relationship is described as love, the marriage is beyond repair.