So your theory is, they lack the intellect to be faithful?
In essence, yes.
Commitment, dedication, empathy, honor, morality, contentment. These things are not found in children nor in those with the minds of children. It takes a certain amount of cognizance to understand concepts such as these and to see them as more than just vague, undefined notions. Lacking the necessary maturity your wife, and mine, simply do not fully grasp them, they cannot.
Have you ever known a child that was fully content? Children soon tire of the toy they have and want another one. They move through childhood always wanting the next great thing and soon after they get it they're on to the next thing. Some people grow out of this state and mature into adults but many more do not. They always want more, bigger, better, faster, newer in their effort to find contentment, never understanding that contentment comes with maturity and the ability to accept and appreciate what you have as enough.
And, although some people do experience measurable growth throughout their life, attaining some wisdom, many more simply do not. They are forever enslaved by their adolescent minds never really having control over their destiny. They are forced to react to the consequences of their bad decisions rather than proactively making good decisions thereby thwarting the negative consequences.
Science tells us that mental development ceases in our mid twenties therefore, whatever synaptic network one has at that age is by and large what they will have for life. Change is rarely possible but highly improbable. Regrettably, this is why the phrase "once a cheater, always a cheater" is nearly always accurate.