From time to time manga will throw really gross shit our way, but I think at least for this one the character was putting himself in the shoes of the uke. It's kind of 50/50
Yes that sentence was really bad, but at the end the barkeeper is proved wrong right? We see that he is prejudged and sadly the world is full with people like him thats why its good that we have characters like Maruyama who will prove them all wrong. Sometimes Mangas can get way to realistic but i dont think the message of the Mangaka in that scene was that Bisexuals are not trustworthy but the opposite she wanted to show us how hard they have it even in the lgbt community.
I was enjoying this manga but that "you can't trust bisexuals after all" line...it hurts as a bisexual person how we're percived as cheaters time and time again. Anyway, not for me anymore