1. They have a casual fling. Maybe something that happened one drunken night at a conference. They confess all. Apologize. Swear it meant nothing and will never happen again.
2. They had a long-term love affair. Fell in love with a co-worker or something, carried on for a year or so, but it’s all over now. The other person is gone. They Apologize. Swear it will never happen again.