ZOMG it's me. Yes I sidle in from time to time. This topic happens to an area of expertise:1. What would you do if you found out your partner/spouse had cheated on you?
Kick him out. Make him sign over title of the house in exchange for a used Jeep Cherokee. Get a divorce. Live well.
2. Does it matter if it were a long-term deal or just a one-night fling?
Nope.
3. Does your opinion change if alcohol/drunkeness was a contributing factor?
No.
4. What if it weren't the first time?
It wasn't but even if it had been betrayal is non-negotiable. See my clarifying remarks below.
5. Any other thoughts/questions/comments?
If two consenting adults agree that monogamy may not be possible and that outside relationships may be an option as long as they are agreed to beforehand and precautions are taken then there is never a reason to do anything on the d/l. The fact that he did so anyways on multiple occasions was proof enough that he didn't deserve me.