Sorry cobra have to disagree that mlc does not exist! Yes I think that there may have been problems with R issues that have not been addressed but in mid life they come screaming to the fore!
Both men and women evaluate what lies ahead in mid life and the things lacking in their relationships or work come to a head.

This often results in a man having an affair maybe because he feels the spark has gone in his M ie. less sex or not being appreciated at home or feeling work is just a chore and he hasn't done all the things he dreamed he would do or he just wants to feel free again. I say he but this could be the woman too.
While lack of sex could be one of the issues it is not always the case you only have to look at some of the people on the mlc board to realise that.

From reading some of the posts here I realise why my H had an affair 15 years ago and can see things more from his point of view BUT my point of view is different. Yes I didn't initiate sex so often because I was exhausted with a young baby and a H that did absolutely nothing around the house and I mean NOTHING!!! Sorry but I didn't want to do much other than sleep and didn't care about sex. So o.k. he may well have felt frustrated and thought I didn't care about him well boo hoo! Why couldn't he have done something more to help?
If he had booked a weekend away so we could have had time alone and had explained he was frustrated I would have tried to change but no he chose to take someone else out for dinner and to hotels for sex.
I have read books on what makes a man tick etc. but the reality is most men don't try to fix what they already have they just look elsewhere.
I've thought a lot about things I've done wrong but I would bet my life my H hasn't for one minute considered what he might have done to contribute to any problems in our relationship!

I absolutely agree there has to be a build up of resentment and lack of communication before a M/R breaks down often caused by lack of sex/inimacy but it is never 100% one person's fault. You can read all the books on the planet but they don't help when only one person is taking notice of what is written and honestly most psychiatrists I've met are insane and psychologists aren't much better. Believe me with a son with serious mental health probs I've met quite a few!!!

My heart breaks for every man and woman on this board that wants nothing more than to feel loved both physically and spiritually by their chosen partner and is searching for a way to achieve this. Good luck to us all!!!