I'm reluctant to wade into this minefield, but I think Maya identifies herself as a lesbian based on more than just her time with Josy. Note her exchange with the MC in Episode 5:
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She says she's only seen that with
girls, not with
Josy. To me, that suggests that Maya has been attracted to girls other than Josy, even if Josy is the only one she ever slept with.
Given her family background, it seems very plausible that Maya had a long time to recognize that she was attracted to girls rather than guys (and agonize over that fact) throughout her teenage years. So she was convinced she liked girls in general when she finally decided to try anything overtly sexual with Josy in particular.
The MC is therefore notable because she had similar feelings for him despite him being male. Technically, being forced to grind on him is overtly sexual, but it seems clear she felt
something about the MC even before that. The tazing incident was the catalyst that made her realize exactly what those feelings meant.
Hence her confusion. She felt an attraction to girls, and her activities with Josy are accepted as proof of the general case. She felt an attraction to the MC, but not to any other boy. So how much do her activities with the MC prove?
That's my two cents, anyway.