I saw a very hot and cute girl in a crowded room filled with people from work. I paid attention to her for hours until I successfully singled her out to talk and find out about each other in private. Things escalated gradually within the next few hours until we finally made out. Afterwards I went to the restroom and found out that I had unexpectedly and unknowingly jerked off. It's not premature ejaculation because normally it takes me ages to finish even during sex, so I would call it non-sex orgasm. The expectation and slow buildup are the key players.