It is the weirdest thing how couples, having just had sex, are wearing underwear.

There must be some code I do not know about. It is ubiquitous

Well, it's still better than 50s and 60s, when it seems married couples slept in separate beds (a scene from "I love Lucy"):

https://i.pinimg.com/originals/ac/e1/f9/ace1f9037208231eebf5...

Without meaning to undermine your point, I think a greater proportion of married couples did sleep in separate beds back then. My grandparents had separate beds.

Or not taking off their shoes...

What's with Americans wearing shoes on the bed? we're always shocked when we see that on TV.

I've always wondered about this. They just get into bed, clothes and shoes and all, despite having been outside and walked over all sorts of places. Why don't they at least take their shoes off? It drives me crazy, I've never seen anybody do this.

I think this stems from two things:

1. Americans are always on the run, living at much higher pace, ready to take off at moment's notice.

2. The US is located more to the south than Europe, so the climate is warmer and drier in most parts, so your shoes are much cleaner (no snow, no mud).

Or maybe it's just a movie convention: like they want the movie character to be able to take off without wasting time on finding their shoes. Similar thing happens with how phone calls are shown in movies: there's a very short greeting, and after the conversation is over people just hang off, no pleasantries at the end, in the interest of air time.