Cheating has nothing to do with maturity and everything to do with evolutionary psychology. Men by nature seek to expose their genes to as many different hosts as possible; at the same time, he chooses a superior single mate that will have multiple (or surely one) children. Therefore, he will have children benefiting from genetic variety as well as the choicest traits of his primary mate (eventually). Abstaining from this impulse is a difficult task, especially if sexual incompatibility is involved. Imagine not masturbating (or having sex for the more fortunate among us) for a week or two. Now think about not masturbating for months on end, maybe just on your birthday. And yet daily you are bombarded with porn, tempting you, never relenting. It's not surprising to me that so many people end up cheating.
On the first point, I don't think many people would be incredibly receptive of their wives saying they suck in bed, or husbands that their wives are not serving them enough. If they manage to work it out, great for them, but it isn't just a matter of frequency, duration, or simple matters like that. Physical attraction is a vital part of a relationship. In our day it is often overlooked or even oversimplified as men liking only big boobs and women only liking Abercrombie and Fitch models. What a given person is attracted to varies; I for example like fair-skinned girls with long brown hair. You may like suntanned blondes from Miami, or perky Asian girls from Roppongi. In addition to a preference, which is typically inherited from parents as well as your culture and upbringing, most people (men mostly) also have a taste for variety, a love of something different that sets you off. Maybe you love Asians, but one day you meet a fine black chick at the mall and head to her house for the night. The point is that those attractions are there to motivate you to have sex. That's the simple way it goes. It's true you can be attracted by another's personality, intelligence, etc., but unless you do sleep around a little you may never find out what gets your motor running long and hard or what is merely a passing fancy.
>I just believe you shouldn't even try it, unless you're sure the person you're doing it with is the one, as it's the highest form of intimacy there is. And you can't really say the person is "The One" until you're willing to marry them.
Sex is the highest form of intimacy? How about sharing your feelings, your thoughts, your dreams and secrets with the person you truly love? Isn't that infinitely more intimate than copulating like monkeys? Seriously, sex is used for bonding between social animals, from bonobos to penguins, conditioning us just like Pavlov's dogs when his wife visited. But we're more than just simple animals; we also have minds that make us as much or more of who we are than our base instincts. We can do infinitely better that just sex. Even though sex is a key part of a durable relationship, a fulfilling relationship also includes so much else, like sharing experiences.