@MyFirmQuoter
I think there are two types of cheats.
The 'habitual cheat' needs no reason to cheat. They could be married to someone who looks like <insert fav movie star>, keeps house like Martha Stewart, parents like 'Super Nanny', and greets them at the door with a drink and slippers, ushers them to their easy chair, and says "Don't lift a finger, Darling. You work so hard, leave the rest to me" and that person would still cheat. They thrive on the thrill.
There's the (for lack of better words) the 'emotional cheat'. This is the one who is unhappy in their marriage for some reason and is looking for something to 'make up for it'. But you never solve problems inside a marriage by looking for solutions outside the marriage. I think this is what you're referring to when you talk about 'reasons' or 'deficiencies'.
So yes, there are things that may happen in a marriage that make someone want to cheat and to justify cheating to themselves. But it's still wrong. It can't be justified by "I don't want to break up my children's home", "I can't afford to leave", "He/she 'let themselves go' or 'doesn't fulfill my needs'". Nope. No justification whatsoever. Not even being cheated on justifies 'cheating back', nor does abuse. Get the hell out, but don't cheat.
Always, always leave with your dignity and your self respect intact.