Hey there. I'm glad that you found us for some support while you're trying to process this.
Bluntly, its a matter of what YOU think about this revelation. Can you get over it? Of course you can. Is it a deal breaker? Well, for you it may be. Some people can reconcile with multiple, year's long infidelities. For some person, a single kiss is a deal breaker. There's no right or wrong answer, just what works or doesn't work for you.
Me personally? I'd be inclined to work through it given my history with long-distance relationships. But I do mean "work" through it. Get to the bottom of why he thought it would be OK when you two had something going on and haul all of that stuff into the light of the day to examine it. Otherwise, you really can't say that it will never happen again. (((hugs)))