I did not recall any scenes that were that sexual where it received the raiting it got.

-The Visitor (Stamps's character) is involved sexually with ALL the members of the family, including the maid.
-Stamp has a brief nude scene.
(Spoilers)-There's also nudity towards the end of the film.
-There are several shots of Stamp's crotch (with his clothes on).
-A couple of homosexual scenes (not graphic).
-And I suppose the religious undertones of the film (Stamp's character, the maid performing miracles, etc) interwoven with sex scenes was pretty controversial for the time.