It saddens me when people make those kinds of mistakes ... no no, not the one where she slipped her tongue down some guy's throat (which most likely was done in a drunken haze) but..I'm talking about the one that causes more damage ... the lying about it afterwards, for months, years, sometimes decades. They rob the person of the feeling trusted to deal with reality & truth...
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Needless to say she should have told u right away but I guess she was afraid of the potential consequences of a stupid move (not an excuse just an explanation) so what to do?..If truth be known (and if she's not a habitual maker-outter when she's out with the girls lol) she's probably beat herself up about it more than u could have ever, she's likely been on her best behavior ever since (not wanting to risk losing u) .....
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So do u trust her?..Well what the alternative?..But use this opportunity to talk about trust and honesty ... try and rebuild it better than it was...
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Don't retaliate that's a waste of energy..

She did a stupid, impulsive thing. Then she compounded the problem by accidentally-on-purpose forgetting to tell you about it. But you can surely imagine why she avoided that conversation, right? "Hey, honey, we really need to write the thank-you notes for our wedding gifts...and oh, by the way, I made out with some stranger last night." ..
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If she hasn't given you other cause to mistrust her in the intervening years, I'd cut her some slack. After all, you've surely done some things that you're not proud of and don't care to admit to your wife, yes? And your wife clearly thought that you'd be OK with this knowledge now, so many years later, which speaks to the trust and respect your two have built in your relationship. Don't destroy that because of one drunken misstep...

Let's turn that around for a moment and ask you if there is anything you're holding back from her in your latent perfection?..
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As adults we are not programmed to broadcast our faults & imperfections. We do things when we are drunk that we are not particularly proud of and wouldn't want our loved ones to know about, because it might change their perception of us (a perception we've been carefully grooming for most of our sober lives)...
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Trust cannot hinge on a single incident... what about the rest of your life together ? Has she given you reason for concern ?..
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C...
Angie's glass house...

Hmmm. That is sad. I wonder why she did it -- and also why she didn't tell you for a couple of years. I have a friend who on her wedding day made out with her maid of honor. But, her husband was right there. And they've been together like 15 years...
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It is a violation of the wedding vows and of trust. Did she just kiss him? Maybe she had a moment of missing being single. Maybe she was wondering if she should stay married (looks like she did)...
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I wonder how this came up and why she told you now......