Everyone being gay is a trope not only in BL manga, but in slash fiction in general. Rather than wondering whether it's realistic or not, it's far easier to handwave it and just understand that the author wants to write about BL couples and it's easier to include multiple in one series than to try and get several series greenlit at once.
Also, on the subject of Ken... that boy is very deep in both denial and internalized homophobia. He's come to terms with the fact that he likes Kirara, but he can't wrap his head around the idea that maybe Kirara doesn't have to bottom just because he's smaller and cuter. He also can't accept that he himself is on the spectrum whether he likes it or not. They're hurdles that he's going to have to overcome eventually, because as the years go by, I doubt Kirara will fine with him maintaining the bullshit he is now. However, does any of this make him a terrible person? No. He's a teenager with ingrained ideas that his very sense of self is rebelling against. It doesn't make him terrible, it makes him a little tragic.