Your wife violated two rules: One, she didn’t tell you when she should have told you. Two, when she told you, she told you too much.Notice that at no point does Dear Prudence say that the woman should have told the truth, the whole truth, and nothing but the truth. Being solipsistic, women are always eager to protect their ability to hide their own secrets from men. But the fact is that any woman who behaved like this because "she felt sad" is almost certainly going to do it again, particularly one who exhibited no respect for the institution of marriage when on the other side of it. They've only been married for two years, and it's quite obvious that the distressed gentleman is a Delta fall-back scenario for the woman; the fact that he feels "quite lucky" to have her suggests that she's the most attractive woman who has ever been nominally interested in him, or at least in a relationship that involves him supporting her. Imagine that.
But you now need to put what you’ve learned into perspective. You two have been together for five years, and you don’t indicate that during that time you have ever had cause to doubt your wife’s fidelity or her satisfaction with your sex life. I don’t know if meeting you was the turning point in her life, or if by the time she met you she had moved past her emotional problems and was ready for a more fulfilling relationship. Whatever it was, it’s clear that when you got together she was a different person from the one who sought out illicit and even degrading encounters. You must know that people do change and that many people are able to leave destructive habits behind for good.
Your wife was not obligated to spill all to you when you were courting. But at some point after you two became serious, she should have informed you to some degree about her past, enough to convey the salient point that she once went through a difficult period during which she “self-medicated” through sex. She could have mentioned that she’d slept with married men and been unfaithful in previous relationships without going into detail. It would have allowed you to have a sense of her past without having disturbing images seared in your mind.
What’s important now is for you to remember that your wife is the same person you’ve known for the past five years, and that there’s no reason this confession should cast a shadow on your good fortune at finding each other. It would be sad if her desire to have you understand her better leads to your loving her less. You just recently got this news and have understandably been ruminating on it. Now it’s time to stop. See if you can decide to push these thoughts out of your mind and make the choice to return to being grateful for your life together.
It's true, people do change. But they change for the worse as well as the better, and the scenario he is in is so common that there is a name for it: Alpha Fucks and Beta Bucks. It seldom ends well even when there is a considerably less sordid backstory. There is a good reason he's having trouble accepting the fact that he married under false pretenses; he should follow the lead of the gentleman who belatedly found out about his wife's college threesomes and Next her without looking back.
There will always be reasonable exceptions, but in general, young men should start expressing a firm No Disclosure Means Divorce policy, as this is the only way that women will begin coming clean and permitting men to make informed decisions with regards to whom they will or will not marry. In law, willful failure to fully disclose amounts to misrepresentation and is grounds for negating a contract, and the same principle obviously applies to marriage.
Wikipedia: Misrepresentation is a concept in the contract law of England and some other Commonwealth countries, referring to a false statement of fact made by one party to another party, which has the effect of inducing that party into the contract. For example, under certain circumstances, false statements or promises made by a seller of goods regarding the quality or nature of the product that the seller has may constitute misrepresentation. A finding of misrepresentation allows for a remedy of rescission and sometimes damages depending on the type of misrepresentation.