I wouldnt tell him... you have a wonderful, chamed relationship as you put it, and you know you're an idiot.
I found out someone significant had crossed me, about two years ago. Apparently only messages, nothing more, I have never ever looked at them the same way. It had ruined everything. They're still in my life, but the paranoia is always there, the how could you, hits at stupid times eg. We are out having a wonderful time, and these thought will consume me, when I should be enjoying the moment.
If it was just as simple as a kiss, or messages, that didn't accumulate to a huge affair, then I absolutely wish I didnt know, and could still view that person the same way.
Now.... if it was a huge affair, with emotions, sex, long term then I would absolutely want to know, and get out ASAP.
My opinion is not going to be popular, but in this instance, you know you're a twat, you know it can't happen again. So go above and beyond to make sure it never does. If there is a next time, you absolutely must tell your DH.
He deserves better anyway, honestly, but if you think for a second your relationship will continue after he knows, I suggest you don't tell him. I honestly wish I didn't know about my person's "misdemeanours" ....it opens up a whole can of worms of if your capable of this, what else are you doing?
So in my opinion, of your situation, i am saying not to tell him, but never, ever put yourself in a position where something could happen again.
No drinks with colleague, no alcohol if it can't be avoided, I'm guessing your not chewing his face in front of the whole office, so never, ever any alone time with this man. Its quite simple to not put yourself in the situation. Avoid and where you cant avoid totally, don't put yourself in a situation where it can happen - eg. Being under the influence.