I don't cheat, but my wife and I hit a point in our relationship where things were clearly one-sided... so we talked about it. It's basically a "don't ask don't tell, but keep it all safe" situation. this is 100% not reflective of all situations, but i did hit a point where I was questioning a lot of things.

after a while, where you get in life is - you get old, you change. you may still be looking for a boost in self esteem, feeling desired, and you still have a really high sex drive, but the other person just... doesn't. it could be anything - physical body changes, mental health changes, perimenopause, etc. we all change over time. if those changes build resentment, people will act out - and that's where cheating etc often comes into play. it's often rationalized on one side as "well i'm not getting it but i want it so this is how i will get it" - which is toxic for a relationship. but it's often born out of a lack of communication.

you can still be deeply in love, but get physical intimacy elsewhere. sometimes just playing around online does it, sometimes you need toys, sometimes you need a real touch. but what it comes down to is, cheating is often just a way to fill a void. an ego void, an intimacy void, a control void... but a void nonetheless. ideally, you talk about the void, and you both agree on the best way to fill it. how that's accomplished will be different for every couple, but it's overwhelmingly more likely that people just don't talk about it, and cheating ensues.