At the risk of totally infuriating some of the regulars here, I would suggest that the answer to your question is: yes, possibly, depending on the circumstances.
For instance (keeping details as vague as possible for obvious reasons) I know a few couples where I would be AMAZED if the husbands (in these cases it is husbands but I am sure there are women in this situation too, although probably not as many) were always faithful. They have been together for years and years, have children, lovely homes, lots of money. The husbands are charismatic, good looking - and I think that, on one level, the wives have decided to turn a blind eye. They think that, rightly or wrongly, they have already landed a catch and the deal may be that their husband has the odd mistress now and then.
In some other cases, I know the husbands have been unfaithful (they were indiscreet enough to tell a close friend) but they get away with it probably because the wife choses not to find out, and does not want to have to deal with the consequences of a discovery.
Whether people like it or not, there is a lot of bargaining going on in long-term relationships, and some people would rather maintain the status quo than split up over an infidelity, with all the losing face/hassle/aggro etc that is involved.
Not saying it is right - just that people have different deal-breakers when it comes to relationships and what is a deal-breaker for one person, may not necessarily be for another, when all other aspects of the relationship are weighed up, especially where children are involved.