Spermula

Spermula is a French softcore science fiction film from 1976.
The planet Spermula is facing destruction and the Spermulites plan to relocate to Earth. To do this, they have come up with a brilliant plan. They will transform themselves into beautiful women and suck out all men's semen, thus making men tired and lazy from sexual exhaustion and unable to procreate.
One of the Spermulites didn't transform as planned... Instead of a beautiful woman, he became a beautiful man, Werner (played by Udo Kier). He likes being a man, except for one problem. His penis is only one centimeter long.
The spermulites are cynics who hate this disgusting love and sex thing that Earthpeople like so much. But some of the spermulites actually start enjoying themselves and think that maybe love and sex aren't as disgusting as they thought.