Blue Magic (1981)

Candida Royalle is a woman with a past a long, long past. Rumor has it she's lived for a few hundred years. The question is how she's managed to remain such a luscious piece of woman flesh after three centuries. That question gets answered when she invites three couples over to her ritzy mansion for a dinner party. Things quickly slide from cordial dinner conversation to kinky wife swapping and group groping. The secret to eternal youth, it turns out, is sex, sex and more sex.