The answer is really a subjective one. When a man cheats on his wife, or a woman on her husband, the problem is not so much the act that took place as it is the breach of trust – one of the parties broke the implicit promise made when they established a committed relationship. Most people understand that a person can be cheating even if there was no sex involved. If a married man takes a woman out on a romantic date, buys her dinner, and kisses her goodnight, is that cheating? I think most people would say yes.
But since the core of the issue is the breaking of the agreement (spoken or unspoken) between husband and wife, the question of infidelity depends on what the couple understood that agreement to be. (Note: this is a good reminder of how important it is to communicate so that both people have the same understanding of what it is they’ve agreed upon!) Some couples have an open marriage in which outside partners are allowed – hard to call that cheating since both partners have openly okayed it. On the other hand, I once worked with a client who had an agreement with her boyfriend that they could be involved with others outside the relationship, but only with people of the same gender – i.e., she could date women, but not other men. Had she gotten into a relationship with another man, that then would have been considered cheating
So what about sexting? The answer depends on whether the sexter’s significant other considers it a violation of the relationship to exchange sexual messages or dirty pictures to someone else. I think most people would see it as such, and in the absence of an explicit agreement otherwise, I think the sensible conclusion is that it is indeed an act of infidelity. Would you be comfortable with your partner doing such things without your knowledge? Probably not.
When someone suggests that sexting is not the same as adultery because there was no physical contact, or they were not actually with the person, or some other explanation, they are trying to rationalize the problematic behavior. Cheating hurts because it sends a message to the partner that the cheater can’t be trusted; that the partner is somehow not good enough; that the other man/woman is better/more attractive/more exciting than the partner; and many more hurtful messages as well. These messages come across loud and clear whether the adulterous act happens in a motel room or on a smartphone.