According to one porn director, Right Through His Pants in pornographic works is partially intended to ensure mainstream porn is uninteresting to women. The theory is that the average straight male porn viewer desperately, desperately prefers that his partner NOT enjoys porn. Her enjoyment implies his inadequacy. The director says this is why they hire extremely ugly guys like Ron Jeremy: so guys won't feel insecure. Female nudity is great (remember, Most Writers Are Male), and if you can slip it past the censors, imply it through tight clothing, or out and out show them nude — fantastic. But lord, don't show men naked; flashing a penis is likely to blind someone and drive the viewer to suicide. (Margaret Cho has an alternative theory: "[Straight men] don't want a fraction of an inkling of a chance that they might actually get turned on by the guy in the movie and accidentally have a homo moment.")
Admittedly it is exceedingly rare to get a clear view of actual female genitalia, except in outright pornographic material. So during sex scenes in movies or comics, we'll get to see almost all of the woman and nothing of the man but some pecs and a groaning face — if that. Also, nipples might show through a skintight suit on a woman, but the male will rarely have a bulge, even in tights (and usually won't look like he's smuggling a gun carriage), leading to the impression that all men in fiction are Ken dolls. Which leads to another Double Standard.
One theory states that this is why the Yuri Genre is popular among men and the Boys' Love Genre popular among women; there aren't any characters of the wrong gender involved, so you can just sit back and be attracted to both of them (popularity of lesbian porn explained on Coupling: "It's sex with a greater density of women"). Another theory that explains why the Boys' Love Genre is so popular amongst women and men is that it is one of the few genres that show men naked at all (usually arising from there admittedly being very little attraction for straight women in mainstream porn because of widespread instances of this trope).note
Another alternative reason for creators keeping characters clothed during or after sex is to decrease the chance of distracting the audience from important dialogue or other actions. For example, let's say two people are shown making love and they finish and lay back and the woman has huge breasts and the guy would make John Holmes feel inadequate (we'll make this example a Euro art film). They then exchange some important character and plot-related dialogue. Perhaps even "the reveal" that the entire film is pinned upon. Yet viewers are too busy gaping at the exposed genitalia (perhaps they might be shocked by it, as suggested above) - or, worse, they might choose that moment to go buy some popcorn - and the moment is sabotaged. The Modesty Bedsheet can be seen as having the same rationale. The sex act had to be shown, but afterwards the story needs to continue moving along and it can't have the audience fixating on nudity.
There's even another possible reason why some filmmakers choose this over full-monty lovemaking: not every director or actor appreciates seeing their nude scenes recirculated forever on porn and celebrity nudity websites. While it doesn't stop it happening all the time, a scene showing an actress wearing a granny bra (see the trope page lead image) is less likely to get bounced around the net than if she was topless. This goes for full-frontal male nudity as well, perhaps even moreso as it is (rightly or wrongly) less common in film and TV.
