My wife (now 29) once had a fling with a woman when she was 21.
Me and my wife met when we were 19 at university and got together not long after that. In our final year of uni we lived together in a house with 2 friends. One of these friends was a girl who was openly gay. She was quite attractive, dark hair, brown eyes, very large breasts. Whilst we were living together she had ended her relationship with another girl and her attentions seem to turn to Lucy (then my girlfriend now my wife).
They had always gone on nights out together and danced together with a bit of touching but nothing too far, and I was always fine with this. When this girl had broken up with her girlfriend though, these nights out became more frequent and she often tried to kiss Lucy or go a bit further. Lucy would let her do some things but stop it when it went beyond just messing about.