In my ethics courses, I discuss issues in family ethics with my students related to marriage and parenthood. We often discuss an essay by contemporary philosopher Richard Wasserstrom, "Is Adultery Immoral?" I believe the answer is clearly yes. The point of the essay is to get us to think about the arguments and consider some alternative points of view. Here I will simply raise some of the reasons for thinking that adultery is wrong. I will then close with a personal thought.
First, adultery involves breaking a promise, in fact, one of the most serious promises we make. Spouses have made a promise to have an exclusive relationship with one another, which includes abstaining from having sex with other people. When this promise is broken, it is painful for many reasons. Adultery indicates indifference on the part of the offending spouse. He or she doesn't care about the other spouse or their feelings. It may also be seen by the offended spouse as a sign that the adulterous spouse has feelings for another, which can be very painful.
Another reason for thinking that adultery is immoral is that it involves an immoral form of deception. The offending spouse may tell direct lies to cover their tracks. But he or she is also deceptive insofar as the promises made when the marriage began included being faithful. The offended spouse has reason to believe that the promise is being kept. Hence, not telling him or her is equivalent to lying.
A final reason given for thinking that adultery is immoral is that it weakens the institutions of marriage and family. Given that these institutions are good for society, engaging in activities that undermine them is morally suspect.
Much of the above is fairly obvious and commonsensical. But such thoughts may go by the wayside when people are faced with the choice of being unfaithful to their spouse. A student of mine recently said the words I opened with above: Spouses should put their marriage ahead of their personal "happiness". There is wisdom in that thought.
There will be times when you sacrifice your own happiness for the sake of your spouse, and for the relationship itself. However, when we put the interests of our spouse and the well-being of the marriage relationship above our own pursuit of happiness, paradoxically we experience a deeper and more lasting form of happiness. This is the form of happiness that comes from having good character, and an enduring love relationship with the person we've vowed to be faithful to, 'til death do us part.