Ok finally there was some kinda reason or desire for Asuka doing stuff with this girl(she pictured her mom right?).
The main reason Asuka does things with Miyuki is that it feels good. Miyuki is very attractive and very skilled at that sexxor stuff, and Asuka is eager to learn and finds the lessons very pleasurable.
Asuka isn't "cheating" on Ayako. At this point in time, the two are not in a romantic relationship. You don't really have any obligation of faithfulness towards someone who is only a secret crush.
In the end this other girl shivered after realizing whom Asuka was thinking of while getting intimate. Hope it wasn't out of some kinda arousal or maybe her trying to use that as a bribery what
Eh, I don't know. I kinda hope that they will go all the way and that Asuka will learn lots of useful stuff to try on Ayako later. Remember when Asuka gave her a deep kiss for the first time? I wanna see more scenes like that...