Possession (1981)

Mark (Sam Neill) finds out that his wife wants a divorce.  He can’t deal with that news and becomes obsessed with finding out why.  Turns out the lady is a tramp with a new stud.  As he’s dealing with that, he meets his young son’s teacher who is the spitting image of his cheating spouse.  How handy.  They start hooking up and Mark begins to discover that his wife’s love life may be more complicated and terrifying than he thought.

A young Sam Neill is skinsationally skinny and sexy as the put upon husband.  His bulbous butt begs for attention during a hallway scene that also gives a very brief, dark hint at a peek of package.