Actually, rather than her being dense, it's more accurate to say that Honoka likes to keep things to herself. (I KNOW. Not like Honoka at all, right?)
I mean, being dense is one thing, but she agreed to dating Kotori as well, not because she's an idiot, but because this is Kotori, her very best friend. A part of the childhood trio. What would happen if she were to reject her? That means Kotori won't stay with them anymore. Kotori knows that. She used Honoka's weakness for her friends against her. Honoka was forced to accept because she doesn't want to lose her first friend. At least, until the one she loved said that she just wants to be with her and no one else. (which, is pretty fucked up considering how close the childhood trio is)
One thing led to another and after Honoka's rejection, Kotori feels betrayed. She wants Honoka all to herself. She doesn't give a damn about Umi.