One's tempted to answer the question -- or a revision of it (why do men sleep with other women -- or men?) -- with Sir Edmund Hillary's response to someone who asked him why he wanted to climb Mount Everest. "Because it's there," he said. Sexual opportunities are out there, too.
And while there are myriad reasons one might stray, the authors have identified a biological one.
[O]ne factor we think deserves more attention is the role of testosterone (T) in middle-aged men's eroticism. In their twenties, men's T levels begin a long decline, often experienced as diminished passion and appetite for life. Suppressed T levels are associated with depression, heart attacks, dementia, and overall mortality rates from 88 to 250 percent higher. One of the few things that can reliably and immediately revive a man's sagging testosterone is exposure to a new woman. One researcher found that even a brief chat with an attractive woman raised men's testosterone levels by fourteen percent within minutes.
Ryan writes that men may confuse this testosterone spike with "love" and make decisions they later come to regret. But that seems to be straying from the scientific into the moralistic. There's no reason to think this spike triggers feelings of "love." It may be simply feelings of titillation, which are sufficient drivers of action.