Men don't usually leave their wives for mistress's because they never intended on leaving their wife.
He’s not looking at his mistress to replace his wife. He’s looking at her to meet the 10–20% of his needs that his wife either doesn’t, isn’t able, or isn’t given the opportunity to meet.
It’s a different side of his personality that’s engaged in the marriage with his wife than the one that’s engaged in with a mistress.
He likes the person he is with his wife and he likes the person he with is his mistress, but they’re different parts of himself.
He wants to stay in his marriage where he can keep up his good image and his normal life, but he’s gotten so used to having a mistress there too, that the thought of losing what the mistress has been bringing to the situation makes him fear a significant loss of his needs being met. That’s why he wants to “figure out” a plan to keep his mistress around to an extent that will still serve his needs. But most likely, his plan will only really meet his needs, more than it meets the needs of either woman in his life.
A really bad analogy, but yet the best one that made it through to me when I needed it.
The kind of man that cheats for variety instead of desperation born from a sexless marriages typically sees his marriage with his wife as having a hamburger for dinner every night.
A traditional favorite, provides protein and nutrients, a dinner staple, and a very normal sounding dinner.
Over time, having the same hamburger for dinner every night gets old. It’s predictable. He’s had every kind of hamburger possible, but it’s all very much the same.
Then he gets the chance to have pancakes (Mistress) instead and it’s exciting and new, nothing like the hamburger, hits completely different taste buds. It’s sweet and indulging. He’s always happy when he’s having pancakes.
He wants the pancakes to break up the monotony of burger night, and for a while it might even seem more desirable than a burger.
However, when it comes down to it, he never wanted to permanently replace the hamburger with only eating pancakes.
That makes no sense to him. He’d much rather have both, and if he ever ended up in a situation where he only had pancakes, after a while he’d want to mix it up with something else, the second he’s craving some sushi or pizza.
That’s just who he is. He wants to supplement to achieve variety, while not having to look like a bad person by divorcing his wife and getting to maintain that level of stability that his marriage brings.
Just an opinion.