Shimizu is full on gay about Maekawa, she's in love, she wants to be her GF, she wants everything that comes with that package. Meanwhile, Maekawa is pent up, frustrated and SUPER LONELY, to the point that she just wants to fuck Shimizu to relieve some of her stress because, well, she's into her? But not really into her too too much? But I mean she wouldn't mind at all if they were sex-friends? Then again it would totally be weird so she will make do with being with her as much as possible because it eases her loneliness?
This is essentially what Shimizu catches onto, that Maekawa isn't in love with her, she just wants to fuck, to put it in crude terms, which is something that Shimizu herself may WANT to do, but not like that. Not with someone that doesn't love her back. And that's the point, Maekawa still ain't in love, but she's getting there, give her some time... and Shimizu is and is fully committed to go through with her feelings (As of this chapter)... but only if Maekawa actually takes her seriously and loves her back though, because fuck "pity sex" sessions with your crush, it will NEVER go good for you. Ever. PERIOD.