Or a married couple? "Well obviously we both used to dry hump the mirror, but finding someone who could be mistaken for our twin to make love to just seemed more healthy. Well, not healthy, but healthier."
Or how about in the future a little where ageing is taken care of quite well and then play mother and daughter? Just on the cusp of the creation of such things where the daughter has to cope with her mother suddenly being as hot and vital as she is? No more cougars? How sad.
Or the victims of polygamy? "It's obvious that your husband had a "type" of woman that he was interested in." And every week they travel America asking petite hot blondes if they married their husband too because they medically need to kick his ass.