Also, can we talk about how terrifying this universe would be to live in? The characters have to act in a certain way, otherwise their entire being would just fade into the background. And the heroines don't seem to be treated much better. In Megumi's case, she's the designated fanservice girl - as stated, she's won't end up with the protagonist and will just have her boobs groped for the manga's entirety. At least she does realize that and start standing up for herself thanks to Misaki, and basically falls out of love with the protagonist.
And speaking of Misaki, Megumi's comment on page 9
You're also a heroine, so you'll quickly start to like the prota-...
Implies that the girls who interact with the protagonist will end up liking him no matter what. That's pretty much always what happens in a harem romcom, right? It makes Misaki being nervous about getting into an "ecchi" scene all the more understandable - if she refuses, she'll end up as an extra. But if she goes through with it, she'll lose herself and be forced to fall in love with the protagonist.
Really, that's what makes Megumi getting in the locker with Misaki all the more satisfying. Not only does she save Misaki from becoming an extra, she also saves her from falling in love against her own will. Not to mention, Megumi comes out of it with someone who actually does like her back (and not just for her boobs).