I think I watched this film in the mid 1990's but it could be also from 1980's.

It is a thriller/monster/comedy type of movie I think. I don't remember most of the details or the title, but I vividly remember a scene about a guy who is about to fight the bad guy but needs to have sex to turn into the monster and some woman has sex with him and he fights the bad guy.

Please help me solve this mystery that has been racking my brain for years.