It has become my belief that in the green world of Sumiko, homosexuality or heterosexuality don't really exist as concepts.
It has NEVER been mentioned that Mitsuki or Aya are lesbians, or that two girls being together is something odd or to feel guilty about. Aya has never been upset or conflicted that Mitsuki turned out to be a girl, only that she had lied to her.
Sumiko has carefully avoided dipping into the topic of homosexuality. Their classmates have been surprised when Aya and Mitsuki said they were dating, but weirdly, the taste in music was an heavier topic than homosexuality (!)
There was no real "gay panic", just "crush panic" that happens all the time in manga to characters attracted to each other.
Homosexuality is the elephant in the room for the readers, but it's like it doesn't exist in-universe.
So the relationship between Aya and Mitsuki, as written by Sumiko, isn't queer or straight.
It's a relationship, period. With its ups and downs as we can see in this arc.