You are talking about the one with J.Depp and M.Brando in it aren’t you?
Aside from some scenes with Depp frolicking with a bevy of beauties, exactly which sex scenes involving Brando (as the only married guy IN that movie I can think of) are you referring to?
I don’t remember any between him and his wife, even though people seem to quote it often enough.
RealityChuck is right. You don’t see the sex (thank God), but you get Marlon Brando’s character coming home from work early to put the moves on his wife. How could you forget the post-coital popcorn catching scene?
I have a theory that isn’t a total answer, but I think is part of the reason. Or rather, two related reasons.
It basically boils down to economy. Most of the time you want your film to be fairly economical in its use of different elements. In other words, you only have so much time to show what you want to show and you want each moment to have an impact. In many cases married sex has less of an impact, it’s probably less exciting and you don’t have to show it since most people assume that happens.
Given the MPAA rating system, you don’t want to show sex just because and get slapped with a higher rating. That is to say, if you are going to show sex it ought to be exciting and it would also be nice if it moved the plot along.
While it wasn’t shown “on screen” (none of them were in those days), what about the famous Rhett carrying Scarlett up to the bedroom in Gone with the Wind?
I would agree that it’s a matter of plot economy. In two hours, you get maybe five major plot points and two dozen minor ones, and everything else is extraneous. So a married couple having sex adds nothing at all to the plot, and is therefore jettisoned.
That being said, I can think of an example of a married-couple sex scene advancing the plot: Just about the first thing we see in Spike Lee’s Jungle Fever is a married couple having sex. It’s a plot point, because the sex isn’t especially good, thus helping to motivate the husband to pursue the officemate.
Also, the series Firefly handled its married couple as well as I’ve ever seen. They’re totally devoted to one another, but there’s still room for individual insecurity creating drama. The “War Stories” episode is the highlight of this.