There are as many reasons for divorce as there are people. In my experience affairs can be a final symptom, rather than the root cause, in some divorces.
Looking around friends, acquaintances, colleagues. I’d list
married too young and grew apart
married because of unplanned pregnancy, it never worked well
both spouses bi and not at ease with it so lurched into an opposite-sex marriage
couldn’t cope with one spouse’s serious illness
shouldn’t have had children, one spouse hated family life
external (bankruptcy etc) stressors killed marriage
Unsuccessful IVF, many rounds, and then one spouse wanted to adopt and one didn’t
Some of these also involved affairs, but not all. In the situation where one spouse just hated family life and should never have gone along with having children, he moved to a rental nearby and lived alone for several years before entering a new relationship. I’m only in touch with the man in the bi couple, but he’s never explored relationships with men (repressive puritanical background), and is now remarried to a woman. I think it will go exactly the same way as his first marriage.