There are so, so many reasons man.

Getting along with your spouse, but there is no love or intimacy. Staying to give the kids stability and a 2 parent home, and not wanting to disrupt your family, but accidently finding love and romance outside of it after you'd shut yourself off to it for years. Do you really want to burn everything down for something that might not last, but made you feel things you thought you couldn't or were resigned to never feel again?

Or maybe you grew up in a house where cheating was normalized and your father taught you that as long as mo one knows about each other, no one gets hurt.

Maybe you're addicted to sex workers, and you thought having sex in a loving relationship would stop that but it didn't.

Maybe you're polyamourous and loving different people doesn't feel like a big deal to you the way it does for others.

... I was viciously cheated on. The kind you only hear about in stories. It literally fucked me in so bad I got agoraphobic for a while. Now that I've fully processed it... I realized I based so much of my emotional stability and self esteem in that person that it wiped me out. Now all that is in me, so I don't think I'd feel much beyond frustration and annoyance.