Annie - there is an amazing article on Infidelity by Frank Pittman. Can't remember the title, and I am not on my usual computer, but he gave a whole raft of reasons for affairs, and most of them are not exit affairs. Woman usually think harder about having affairs, because it means more to them [OK generalisation, bound to be counter examples]. Men in MLC - mild or severe, are looking for diversion, escape from their humdrum lives. They want the excietement of being in love, and the adrenalin rush. Plus they are worried about potency and so on. Lots of people cannot be attracted to two people simultaneously, so if they are having an affair they think they can't be 'in love' with their spouse, and aren't forward thinking enough to see that the affair might end. SInce many affairs last up to 2 years you can see where all this is leading.
I blame the western thinking that focuses on romantic love, rather than companionship, trust, and deep mutual ties that underlie successfullong marriages. Most people who were happily married and divorce regret it [apparently]. We need as a society to do a major rethink on marraige family and divorce. I wouldn'twant to go back to punitive situations, but I would like to see mariage treated with more respect than it is today. Gosh I sound like ann old fogey. But it is a Christian sacrament, and a major life commitment.