I tend to agree. I don't believe in asking them on "dates" while they're in active affairs is appropriate -- at all.
Either way, your decisive moment came when she TOLD you where she was going, who she was going to be with, and what (by implication) they would be doing. And you did nothing.
From that point forward, she was emboldened to continue her affair, with your implicit approval, even if your words spoke otherwise.
