what's with the sandwich??
what was that all about? (during the intense sex scene)
Short version: it was a perhaps not-entirely-successful attempt on the movie's behalf to incorporate a humorous detail from the original novel.
Longer version: during the same scene in the book, Max is groping around mindlessly between the cushions of Nora's couch, and his hand hits something. When he pulls it out, he finds it's a partially-eaten bag of pork rinds. He's horrified that he's receiving that special favor from a woman who not only eats pork rinds, but is apparently such a slob that she misplaces bags of them between her couch cushions. The sandwich seems to be the movie's version of this, but it looks like it struck a lot of people as just odd rather than hilarious.
The tone of that scene in the novel is both funny and hot, but the movie emphasized the hot over the funny...so the moment doesn't quite work the way it should, IMO.