You were right to hide behind a throwaway account. It's honestly quite easy to construct scenarios where cheating is entirely rational. But even the mere idea of such things make people revel with disgust. I don't expect to change anyone's point of view, but I will provide a very simple example to at least frame the mindset of a potential cheater:
"You are happily married to your partner but after a few years your sexual relationship has ceased to exist. What used to be causal, recurring sex has now become something of a burden. One partner constantly initiates, but the other is disinterested or even disgusted. This goes on for years. You discuss therapy, but your partner won't hear of it. Your marriage is entirely functional and even productive otherwise. You think about divorce. But you've really put in a lot in this house. You still love your partner, but they've changed. You have kids and responsibilities and a life together, and yet an emotional and biological part of you is ground down to a fucking pulp. It's very easy to just say "Divorce!" and clean your hands of the whole affair. But you're not stupid. You know your life could be ruined. Everything you've built can be taken away. The trust and relationships you've built could be destroyed. Your partner is almost an intimate part of your life, and ripping them out is like ripping your own heart out. You're not strong enough to do it. It's cowardly. But you're not perfect, and neither is your partner for putting you in this spot anyway.
So what do you do? You risk it all, because you can't figure out any better. You're putting it all on the line for one little pity fuck that you don't even enjoy, that fills you with regret. You are ashamed of yourself. But deep down you still know that no matter how much love and affection and stability exists in your world, you cannot abandon that single, little fraction of your primate brain that just wants to fuck. You can lie and you can pretend all you want. Make whatever concessions exist in this world for the greater good and the happiness of all mankind and all that fuzzy stuff. And yet, at the end of it all, there's still just you. Alone."
I can easily come up with a dozen scenarios. Relationships change. Power dynamics change. The role of sex in a relationship changes. People change. Sometimes it's not even about the sex, but the possibility of sex and the projection of one's own self worth and affirmation of their attractiveness and self-esteem as they battle life's hurdles.
It's so easy to assume a puerile black and white opinion on the internet. The morally and ethically -Right.Thing.To.Do- stares at you proudly with their throbbing member. But people are not that simple, it's just not that easy.