The “heterosexual explanation for this” is that they’re friends. It’s not that difficult. Y'all preach all the time about how platonic affection should be normalized regardless of sex, gender, identity, or orientation, and then as soon as two female characters so much as glance at each other you’re all declaring that they’re in love and getting married next week.
Why does top/bottom preference in fiction bother you? I have a strong preference for one vs the other IRL sexual activity, why wouldn't I in fiction? I've only ever topped twice and didn't care for it at all. Therefore when I read fic, I want to see the person I'm either lusting over as the top, or identifying with as the bottom. It has absolutely nothing to do with "heteronormativity". Complaining about the top/bottom prefs of gay guys comes off very bottom/fem shame-y to me.