People on here saying 'my dad/brother/husband' has never cheated can't know that. You only have to look at the gazilion threads on here when an affair is discovered with the poster blindsided and all 'I'd never have thought it of him. He was one of the good ones'. The whole aim of a successful sexual encounter outside a marriage is to keep it secret.
I work in a business field where there is a huge amount of men trying it on with other women, sometimes successfully - one offs or long running affairs. I know more than one "happily married man" (a good guy) who has had a long term affair with a woman where his wife appears not to know at all.
This is my definitive theory based on years of seeing this sort of thing going on right in front of me:
*There are a tiny handful of men who are faithful. They are either (i) men who shagged around a lot when younger, got it out of their system and then settle down later in life and then fully commit to a family and bask in their reinvention as a faithful & committed person - often they are men who marry very late (Warren Beatty, Michael Douglas (second marriage) types); (ii) men who have a very strong religious or moral conviction (rare); (iii) men who are faithful because of lack of coincidence of opportunity to have sex without being found out At The Same Time as strong lust/chemistry with a woman who is willing.
*There are a small proportion of men who are just shagging bastards who for narcissistic, selfish reasons will try it on with everyone and have a lot of affairs simply because if you ask enough women eventually some will say yes.
*Most 'normal' men will be unfaithful if you have a coincidence of three things - (1) opportunity to have sex without their partner finding out (2) a strong chemical attraction/driving lust infatuation with a woman who is also willing and (3) both of these things happening at a time when the man is feeling weak in relation to his marriage vows. It's less likely the week after a wedding; it's more likely after 15 years of marriage and they've had a row.
If your partner works in a job where there is a lot of opportunity (working late, trips away, working closely with women) the risks are increased.
Plenty of men are capable of saying no if they are feeling morally strong and committed. The longer the marriage the greater risk there will have been infidelity at some point even if it was a one off 'I feel terrible and am not doing it ever again' regret.
I've seen some and know of some unbelievable behaviour - people having sex in the loo at work, the office cupboard and so on.