I think this is often a reason that men and women choose to cheat.
The terms 'lack of sex' or 'not enough sex' seem very dismissive compared to the devastating effects on a person who is in a sexless relationship. Those relationships seem to almost always be devoid of any intimacy either as often the woman (of course it could be the man too) will avoid any kind of intimacy at all in case their partner gets the wrong idea.
Feeling unwanted sexually and undesired by the person they love can have a huge impact on a persons self esteem, mood and everything in general, it's not a small little thing.
There is also a thing called 'skin hunger', the need for meaningful human touch, it can often be satisfied by sex, but isn't necessarily a sexual need. Being deprived of this can also have devastating effects, affection IS important.
People are often depressed, lonely, tearful, suffer from mood disorders, inability to express emotions etc.
Combine the issues relating to skin hunger with being in a relationship where your partner could touch you if they wanted to and choose not to and I imagine that that can cause a considerable amount of pain.
However there are most likely reasons which led up a sexless (or very infrequent sex) relationship, and people can behave in ways that cause a lot of resentment, lack of communication etc...which then eventually lead to the lack of sex/intimacy and all of those other problems.
Often it is extremely difficult, if not impossible to fix a relationship where there has been little to no sex for a considerable period of time. This seems to affect women more than men, once they start to live like room mates or brother and sister for whatever reason all sexual attraction can die even if they love them.
I despise the term 'withholding sex'. I think it's awful. I love sex and couldn't be in a relationship which didn't have a fulfilling sex life. However you can be damn sure that if there were seriously problems in the relationships and he was being a dick I wouldn't be having sex at that time, that doesn't mean I am 'withholding' it, ugh, hate that term!
So my main point is this, lack of sex is a huge factor and explanation to why some people do choose to cheat or feel pushed to it or craving touch/affection/sex from elsewhere, however lack of sex normally arises due to other resentments and problems in the marriage, because no person should feel like they have to have sex with their partner when they are not getting on at the time.
Off topic, but I have 2 children, a boy and a girl. I hope to instill the importance of communicating with their partner in the 2 of them when they are older. I think a massive pattern in the breakdown of relationships shows that the seeds for the destruction of the relationship start after kids are born, the tiredness, stress etc. at this time often allows resentments to build up at this point that a relationship often can't recover from.
My advice will probably be to explain that all of that is normal, to be tired, cranky, pissed off etc. but to not let the issues fester, deal with them quickly as they arise, acknowledge between themselves that this is normal for that stage and work to sort through the issues.
It is far easier to sort through the issues while you still like your partner as a person, while the resentments haven't become cemented in your mind and before things have been allowed to linger for years unsolved!