I just read this, and cannot beat Lothgar's, but to take it from another angle.... What about a confidential email (could be love letter, or exposing some secret) the guy intercepted and sent around (ridiculed), or was sent to him sub rosa, yet he violated that. You could add more drama by making it somewhat ambiguous so the guy doesn't realise what he did to her.
Maybe they met at a party and they were playing spin the bottle. He was embarrassed and went to give her a quick peck, but she thought everyone was going full throttle and started to open her mouth, maybe even had her tongue coming out?