What I find interesting is the implication that Miyo and Aoi are both interested in gender bending. Enough to talk about it in passing anyway, with Miyo blushing. I've always been... wondering what Miyo would think if she found out Mahiro's secret. It's always hinged on what she thinks of gender bending. I figured it'd go one of two ways.
1. "A snake has infiltrated my perfect garden! Trying to interfere with yuri! Mahiro, how could you?! I TRUSTED YOU!!"
2. "... Science has finally advanced to this point. Mahiro-chan, you're proof of humanity's evolution to yuri! We need to get this drug into the water supply ASAP!" (while Mahiro stammers something about how that'd probably just dilute it)
If she's already comfortable with gender bend, #2 is looking more likely.