I mentioned "not trying" which leads to gaining weight. My wife has had two kids, her body doesn't look the same as when we were married. But she exercises and is always trying to look good for me. I never come home and she is still in her pajamas or has she stopped wearing makeup or jewelery..etc. She still "tries" for me. Keeping a marriage happy is doing all the stuff you do when you are in the dating scene and courting your partner.
Like with me I still try to impress her by dressing nice, working hard, exercising, always trying to make her laugh, opening her door for her etc. I still do my best to continue courting her.
I know several friends wives have stopped trying. They don't get ready, gain weight by not exercising. don't wear make up...etc. Some of them have cheated on their wives but it was not just purely sexual. When women don't feel the need to "impress" their man is a kick in the ego. And vice versa Im sure. If a woman isn't getting attention from her husband she makes look elsewhere.
So if my wife stopped trying and let herself go I totally agree that would be really hard. I'm not justifying the cheating I'm just saying both people in the relationship need to make the others feel worthy and wanted.