Wait. Now we've changed the subject to maturity? That wasn't the original topic.
And as far as reasons for cheating, one not able to keep up with the other is still not common. There are usually plenty of other, more important, factors that determine the likelyhood of cheating. It's not going to be common that a happily married couple, still in love, will cheat due to a lack of intimacy. Because cheating suggests an unhappy marriage. Or a dishonest spouse.
These blanket statements you make simply are not too common.