It's a sensible question. My answer, as far as staying sexually faithful, is to find your wife sexually attractive no matter what she looks like. So, you're sexually turned on by her mind. Aslong as her mind stays the same, you'll find her consistently attractive and not need to stray. At least, that's a very simplistic take on it. Whether it could be achieved in reality is a good question.
Alot of people seem to marry based half on looks and half on mind, so when one half starts to go they think about finding that stimulation with someone who has the looks they want, yet come back to their wife because their wife still has the mind they want.