I admit, the frequent references to Kase-san being "cool like a boy" do get pretty old, and seem out of place when Yamada's actively checking out her tits. It's pretty certain that in practice, boys never entered the equation for her.
I feel like Kase is literally the first person Yamada has ever been attracted to, period. Like, would Yamada hook up with a different girl later on down the road if she and Kase broke up? I dunno. Don't have a super strong feel for it either way.
Props to her, though, for not even blinking about being attracted to a girl. She reacted positively to their first kiss and didn't bat an eye. Like, in other manga, when one of the romantic principals says "...but we're both girls!", you get the feeling that they're saying "...but society says this is wrong on some level!" Whereas when Yamada says it, it's more like "I don't even understand how this is supposed to work!" Like, legit confusion over the mechanics of the process rather than metaphorical hand-wringing over the taboo.