The Curious Dr. Humpp (1969)

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

A doctor kidnaps couples who are having sex and takes them to his mansion where they are being kept as prisoners. He injects his subjects with a substance to 'enhance' their sexual abilities. The curious doctor then forces the abducted people to have sex. While they are making love he drains a fluid from them. He needs this to prevent himself from being transformed into a monster. He gets his instructions from a talking brain.