You are right to be upset but ending a long term relationship over a snog is stupid in my view.
I think a lot of women are incredibly naive about men and infidelity. You only have to look at the threads here about MM leaving/affairs being discovered to see the millions of wives who are totally shocked/never thought he had it in him/never saw it coming.
Frankly, all men (apart from a tiny % of those not really interested in sex at all or highly religious) are capable of infidelity. Men like having sex. Sex with new people is very exciting. Feeling desired is exciting. Over the life time of decades of marriage - it's basically a question of whether the opportunity to do it and get away with it (ie. pretty sure wife won't find out - whether that's because it's abroad/with someone who has more to lose/ total stranger etc) coincides with a woman who he really has chemistry with and is also up for it. Decent men do actually have affairs; just because someone succumbs to temptation when you are just talking about - a kiss - doesn't make him the devil.
So in your situation, it's a good thing he confessed and it's a great thing it stopped at a kiss. Kissing is just kissing. Actors do it for work all the time. It's not great obviously but it's not a hill to die on IMO.
Imagine this: you are alone, abroad on a work conference. A really attractive man you've got to know over a few days is flirting with you. He lives miles away and you'll never see him again. He is making you feel very attractive and desired. You feel like a teenage. It's fun. You drink a bit more. More flirting. He has you alone in a romantic candlelit dark corner and leans in for a kiss. Are you really, really sure that in that moment you wouldn't enjoy a fabulous snog? And it really mean nothing at all. Doesn't mean you don't love your partner. Doesn't mean you are running off with this man.
Just means that you enjoyed sexy, fun, flirty kissing with a guy for one night in your life and no one will know?
Maybe I'm more male in my attitude to this kind of thing but I have definitely been head over heels in love with someone and had a drunk snog with someone else that meant nothing.
I'm not saying he should get a prize, or that you should over look it or not give him a hard time. Just that it definitely isn't worth ending a relationship over. That's all.