Every Inch a Lady (1975)

Harry Reems and Darby Lloyed Raines are a couple who have built up a thriving business around each's ability to satisfy lovers of the opposite sex. They started out with more cash doing the kinky stuff. So now they specialize in supplying well-paying customers with customized sexual services. The money's rolling in and they seem to be very much in love, but trouble's brewing. Darby overhears Harry pillow talking with one of their workers with whom he's been spending his off-hour sexual energies, and it turns out Harry and his new squeeze are planning to rub Darby out and take over.