Update: The move list is now inside the game folder. I'll keep this post here only for reference, but it is outdated!
Note: Once the sex move has started, you can change poses with the arrow keys and control the intensity by pressing the punch or kick buttons.
Note2: Some characters have multiple sex moves that can be done by doing the same combination but just using kick instead of punch and vice-versa. Try it out!
Zangief(sf2 ver) - get close to the opponent and press D
Minotaur - quarter circle foward + X
Bao - get close to the opponent and press X
Poison - keep pressing down + Z and X
Hentai Kungfoman - quarter circle foward + X or C
Kuromaru - quarter circle foward or back + Z or X
Kungfu man - get close to the opponent and press C while pressing foward.
Brian - get close to the opponent and press C while pressing foward.
Bison - quarter circle back + A. Choose what the opponent will do by pressing kick or punch buttons.