"His sexual impotence turned him into a horrible monster!"

The Curious Dr. Humpp

A doctor—on orders from a sentient brain—kidnaps intimate couples and takes them to his mansion as prisoners. He injects his subjects with a substance to 'enhance' their sexual abilities. The curious doctor then forces his hostages to have sex. While they are making love, he drains a fluid from them, which he needs to prevent himself from transforming into a monster.