Betting my money that Glasses girl is asexual
That's not how that works. Asexual people don't feel sexually attracted to anyone. The urges in general can still be there and people who lack the urges might still be sexually attracted to somebody, it's not necessarily a package deal where you throw out one and the other is gone too. Many if not most asexual people probably still masturbate, because unless they also have little to no sex drive, they're gonna want that release from time to time like anybody else. Since they're not sexually attracted to others, they might lean towards playing solo more than average.
Some things that could be going on here instead would be: They're reading the basic how to, but maybe she gets her jollies from something slightly more specific and out of the norm. She could have low sensitivity to stimulus like this. Or, perhaps, it's psychological. She might be putting too much pressure on herself here to actually enjoy any of this, and doing this stuff with an audience could be exacerbating that as well.