IMO people (not just men) have affairs because
they think they'll get away with it
no strings stranger sex is sometimes attractive, and then it turns into something more if it's an affair as opposed to a shag
they're flattered
it's a million miles away from the responsibility/drudgery of family life
they don't, mostly, think they'll get caught
sex
excitement, adrenaline rush
they don't really care about the relationship they're in
they are concerned that their partner is unfaithful/will be and so they want to get in first (applies more to younger people I think)
they're insecure, want reassurance re attractiveness
I've been unfaithful in some relationships but never (and never would be) in this one. When I was a young slapper my experience and that of my friends was that men (married and single) rarely turn down no strings sex. But this doesn't mean I think all men are persuadable, I trust my dp and I trusted ex dh and was right to in both cases I think.
When I went out to work last year I totally understood why men are tempted (and sometimes succumb) to an affair with someone who is
well groomed
has no children
has a LOT of energy/time to lavish attention on them
is offering to be fantasy woman in terms of no committment/drudgery/long term stuff
is attentive
is sexy whilst being attentive: How Was Your Day Darling? (as opposed to 'There's a poo in that nappy and ds needs a bath/nits doing/other vile chore here')
is nothing to do with the hard work that is family life
I absolutely wouldn't do it (were I ever to get any offers and that's not likely in the forseeable future, I do realise this!) because I love, adore and fancy my dp like mad and nobody could ever, ever measure up to him. Anyone else would, I'm sure, be a huge disappointment (and, as my sister said "and no-one can say you haven't been around" [reformed slapper emoticon here]) but fwiw those reasons are, I think, why people have affairs. All looks very reasonable and understandable but actually, I do think extra marital affairs are always wrong. I don't think they're excusable and I wouldn't forgive dp if he did it: game over as far as I'm concerned.