To begin with, doesn't everybody "use" the person they try to date to figure out if they want to date them? That's how dating works. Sure, some relationships are more casual than that, but if you're trying something at least semi-serious, you start by dating to see if you're compatible, right? Nobody knows until they try. Luna tried, and it didn't work out.
Even people confident in their allosexual alloromantic identity are going in blind, because relationships are complicated and messy and it's not like you get a manual. People who are on the aro-ace spectrum don't get born knowing where on the spectrum they fall; a certain degree of life experience is needed to understand yourself. From the looks of their clothes in the flashback they started dating as teenagers too (not that undergrads are that much more worldly).
Luna did err in not communicating this, and probably being too insensitive in asking Mai for help in clothes shopping for her rebound date. That's her failure. But failure isn't the same thing as malice.