Some of the picture are quite etchi. For example, the bottom line of the face expressions is NSFW, especially if one happens to be able read kanji.
Meanwhile, I just figured that in the anime, the fact that they ended up at the seashore is part of Miki's joke. The first one was onigiri and the second, as it turns out, has to do with Miki's uncontrollable desire to go to the beach.
The theme is also used in relations manga.
Sorry it took me so long to come up with this (so slow... *bangs head agaist the wall*).