I picked yes.
I would really hate to forgive someone for cheating, and I couldn't imagine being around them if they had done so. Plus I think if a person has weak enough will power that they do cheat, then that won't change, and it's only a matter of time before the chance arrises again and they take it.
However, I love my current girlfriend too much and I wouldn't want to lose her.
But if it happened more than once I'd have to end it.