I think the Subtext tag over Yuri is much more appropriate, because putting aside guessing and wishful thinking, there's no evidence that says it is. In fact, there's more that prove otherwise.
Nothing says that Aya like girls. She had a crush on Mitsuki long before she learned that Mitsuki was a girl, so Aya clearly is attracted to the male form. When she found out, she felt betrayed, even though she didn't knew the reason of the deception. That's because she was interested in a romantic relationship, yet, the fact alone that Mitsuki was a girl was a deal breaker. Whenever someone thinks that Mitsuki is her boyfriend, she dismisses simply by saying that its a girl, because that alone is enough.
"Oh, but the blushing!". The fact that Aya knows that Mitsuki is a girl, doesn't change that she still looks like a hot guy. Her brain knows that she is a girl, but her eyes see a hot guy. It's like watching a horror movie. We know its fake, but our eyes see otherwise, so we feel afraid anyway.
About Mitsuki... we have no idea what she's into. No posters nor magazines or anything that indicates what she is attracted to. The times that she looks hot are completely unintentional, she's not trying to seduce anyone.
Romance ended the moment Aya found out that Mitsuki is a girl. Now its a story about two girls bonding over music.