People have a hard time suppressing their sexual urges. But, many sexual urges we once considered immoral & destructive are now seen differently.
The most culturally acceptable middle class sex life today involves a series of monogomous relationships. We're encouraged to start young with practice relationships (ideally stopping short of "real sex") for teenagers, which are expected to end. Then, a variety of relationships ranging from one nighters to multi-year relationships anyone from the past would have called marriage. Variety is generally valued. Then Marriage(s).
My GreatGrandmother would have raised eyebrows at all this.
Even though sexual norms have radically shifted during this time, our view of fidelity remained kind of static. Promiscuity good, infidelity bad.
The culture surrounding sex is among the most complex in the human repertoire, but such a shift on promiscuity without a shift on infidelity seems like an oddity of our culture, probably a short term one.
By that argument we shouldn't try to understand why anyone does anything we deem negative, and I would hope we can all agree that's incredibly silly.
Understanding "why" helps us understand how to prevent it from happening to people.
This article, in particular, is a great piece for people in healthy relationships to read, because it teaches an important lesson: you are not immune.