Here is an interesting LINK talking about extramarital sex:
And some of what it had to say:
Bright women are 50% more likely than dull women to have an affair. Interesting things happen when one combines attitude and behavior. I call those who both accept that extramarital sex is sometimes justified, and have had extramarital sex, ‘swingers’.
For women being liberal and unhappily married move her from ‘traditional’ toward ‘swinger’.
Other factors do not appear to matter much.
Moral philosophy: It concludes that an affair is wrong most of the time but that there are exceptions.
Deontology says affairs aren’t wrong because of consequences but because it involves violating important principles – like the duty to keep a promise, tell the truth or respect your spouse, or it violates the other’s right to fidelity, respect and the truth.
Virtue ethics says affairs should be avoided because they would smear those qualities that make you a good person – or character – even if only in your own eyes.
Utilitarianism says having an affair is wrong because it can hurt your spouse (as well as children, other family, friends and co-workers). On the other hand maybe you believe you and your affair partner stand to gain more than the sum of all the hurt, or that by not having an affair you (and your paramour) will suffer more than everyone else will gain in happiness.
It appears that it’s silly to demand or expect strict sexual fidelity in marriage at all costs. It is often the sensible thing to do if trapped in an unhappy marriage, and not because it will help to preserve the marriage but because the marriage is probably not worth preserving.
Still, it is also silly to undertake sexual infidelity thoughtlessly. Most of the time, extramarital sex really is the wrong thing to do.