did eri really say she'll never see maina working in the sex industry? what an odd caption
I suppose they're performing in a questionable part of Tokyo (also based off the ChamJam reaction to Gotanda on day 1). She isn't saying anything about "working in the sex industry" but rather that it may have been the only time she will ever get to see Maina in a red-light district. So still kind of weird, but par for the course.
Maina also isn't blushing at her own words on page 21, FYI, there's just one missing in between 20 and 21. Leaving that one out seems particularly cruel, so although I'm not much of a translator:
Panel 1: "Ah"
Panel 2: "Maina might not care whether I'm here or not, but every single minute and second of Maina's presence is essential to my life!!" (flashback to something she said in chapter 1)
Panel 3: "I was thinking about Maina-chan all day yesterday, too"
Panel 4: "I've grown to love you more today than I did yesterday, so I'm sure I'll love you even more tomorrow"