I answered so quickly, that I forgot to add that most affairs are not about sex. Some are, but most are due to other deficits in the relationship, particularly for women.

https://theconversation.com/its-not-just-sex-why-people-have-affairs-and-how-to-deal-with-them-92354

"A survey of 5,000 people in the UK found striking parallels between men and women’s reasons for infidelity, and neither prioritised sex. The top five reasons for women related to lack of emotional intimacy (84%), lack of communication between partners (75%), tiredness (32%), a bad history with sex or abuse (26%), and a lack of interest in sex with the current partner (23%).

For men the reasons were a lack of communication between partners (68%), stress (63%), sexual dysfunction with one’s current partner (44%), lack of emotional intimacy (38%) and fatigue or being chronically tired (31%)."

But, it's easier and more fun to slut-shame women than it is to take responsibility for not being a great partner, which is why so many of these guys go that route.