Brutally honest here, so have namechanged.
I have snogged someone. Been with partner 15 years, never been unfaithful, never even contemplated it. I had my own sort of midlife crisis when I was on my way out of pnd, had managed to lose weight and felt stunning.
I met somebody, was attracted to this person, we flirted, we snogged. I had not been drinking, I knew what I was doing. And I wont do it again. My issues have been resolved now, and I have not told my partner. However, it has caused me heartache and confusion. I love my partner, I love my family, and will not want anything to happen to the family unit.
Actually, it had nothing to do with my partner. It was all about ME, how I felt, and the issues I was working through.
So my question to you. Why did he tell you? He could have gotten away with it. What is it really that is bugging him at the moment? How does he feel in himself? You mention depression. Does he feel unloved? Unattractive? That he is insignificant?
A snog is not something to end your marriage over, i would say it is a sign that something is not as it should be with either him or the relationship with you. I think the best course of action is to try and get to the bottom of it. He has taken the first step by telling you about the snog.
If he told you purely to be spiteful and hurt you, then I would say he is petty and behaved like an arse.
Can you get an appointment with Relate? If he is reluctant to go, go without him to start with (it may do you some good), he may soon decide to join you. Good Luck.