You know, as much as he has to hide his true relationship with Mahiru, he really brings up great points (and he's not exactly lying as he's only referring to his conversations with her at school. It's true for that much, because that's what he's showing to his schoolmates.) Like he's not saying these people can't join the conversation, but they strictly talk about their academics in conversation so if they don't want to actually partake in it and take it seriously, that's on them. They really have no right to be jealous if they don't even got the backbone after all to put in effort.
And especially my favorite, "Objectively yeah, she's cute and has a nice personality. But that does not mean I have to fall for her." THANK clap YOU Amane for being such a realistic and down to earth dude. Yeah a guy can appreciate their girlfriend's (or crush) appearances but if that's the only factor to this, it's just setting up for failure. (also applies vice versa by the way) Honestly I really do love subverting expectations of Amane looking "plain" but actually being so down to earth and realistic. I really would love a friend like him.