That's the sort of dominance/conquering/corruption fantasy Aya likes to picture in her mind while she shlicks... and it's okay, it's her damn beeswax, she's entitled to it and no real people were harmed in the process of reaching that orgasm!
I mean, we don't really know either way, so you may be right, or she may have just chosen this because it fit the theme of her corrupting Marika and therefore would make Marika connect more with her character.
Saying things like "Marika is actually a lesbian and Aya's making her discover it" doesn't change anything to these basic facts; it's just an attempt to sanitize the plot to make it more acceptable for Western pc sensibilities. A really bad attempt, imho, because it seeks to add a correction varnish to something that originally is (and never pretended to be otherwise) shamelessly and barefacedly incorrect.
Well, sure, saying that Marika is actually a lesbian may be a bit much, but at the same time, it's undeniable that she herself admitted finding Aya sexy already before the corruption started, and despite her denial has always reacted well to Aya's advances. So while this clearly is a conquering fantasy, there's also truth to the view that Marika herself is not as straight as she thinks herself to be, although indeed most likely not strictly lesbian.