Here’s a pretty good Newsweek article about it.
Basically, whatever a cheater isn’t getting out of their marriage, they are probably getting out of the affair. If your spouse doesn’t appreciate you, for example, it’s easy to fall under the spell of someone who adores and cherishes you.
Yeah, ideally they would end the marriage first, or have the self-control to “just say no” – but that works for affairs about as well as it did in reducing drug use in the 80’s. People are human beings, imperfect, and sometimes weak. Sometimes the unconscious cost/benefit analysis just doesn’t add up on the spouse’s side. And by the time people are thinking with their hormones rather than their brains, they are oftentimes not thinking at all.