I can't understand why you're not more upset about this OP.
The forethought and deliberate planning that he went into to cheat on you at the first available opportunity is chilling.
It's like:
"I was planning on killing you but I could never get you on your own for long enough to do it."
"Oh well, you didn't kill me so no harm done I suppose."
Also, I don't believe that this was a one off; that the first and only time he attempted this was the time that he got caught. Yeah right. I think that he's probably been doing this for a while.
It's odd, I see this all the time on the relationships board: woman finds massive clues pointing towards an affair, all lit up and flashing in fluorescent neon, but because there's no incontrovertible proof, the bloke gets away with it.
Indeed in this case, the guy has even admitted that he wanted to cheat and would have cheated had he had the opportunity. But because there's no proof that he did, OP is satisfied that he didn't.
I'll never understand why independent concrete proof of an affair trumps admissions of guilt or even ruined trust as a deal breaker.