“Mommy, the man I was sitting next to on the airplane showed me a movie of people doing it, but it’s ok because he bought me a scotch.” 
I guess I’m just afraid of life, but if there is a snowball’s chance in hell I could find myself having to explain my way through something this awkward, I’m sticking with solitaire.
You mean the privacy filter, which was mentioned in the second paragraph and not available (and likely unable to pick up at the airport). I think the OP is aware of their existence.
I fly United a lot internationally and there are quite a few unedited R movies and cable TV shows available for viewing through their entertainment system; I don’t know if they filter out movies or shows with a certain amount of nudity or violence, but there are lots of selections skewing towards mature viewing (I think I’ve seen some Game of Thrones episodes on their system, but I’m not sure).
Sometimes children fly unaccompanied, so mommy isn’t there to distract their child from a next seater’s viewing choices. I think it is good and polite to be sensitive about what the child can see. A pillow or magazine or blanket can be used as a shield of sorts.
If next to an adult, the obligation to be sensitive is much less - the adult can avert their eyes as they choose.
Are you sure they let R-rated material slip thru? I know in-flight entertainment has changed by leaps & bounds in the last few years, but I find it difficult to believe they’d allow that, for all the reasons stated in this thread.
And I’d bet anything, regardless of its MPAA rating, they still do not allow any films involving plane crashes*!*
Um, I’d forego the chance to watch Dexter or whatever because doing so would make my own flight harder. It’s not difficult.
Definetly R rated; I watched a few of them on my last flight last week.