Great season finale (possible spoilers)

They've established a precedent that Frankie likes to make stories up, and the extremely graphic nature of the bathroom incident points towards the likelihood of fabrication.

But why would Frankie not want to come out to her mother? After mulling over it a bit, I think it might come down to not wanting to be treated differently. It's not that her mother would ever judge her, but she would most likely make a big deal about it.

If Frankie is transgender, I get the feeling that it might be a similar situation to the transgender character in Horace and Pete. The transgender woman in H&P simply wanted to be a woman, not a transgender woman, with all the baggage that being transgender carries. It's possible Frankie just wants to be a boy, and not be treated as being 'special.'

Society's long history of parents rejecting LBGT children has produced a generation of parents who feel like they need to compensate by accepting their children in an overly demonstrative way, and, while this is far better than rejection, it's still very gender conscious and still comes from a place of discomfort. The character in Horace & Pete is basically saying that acceptance is better than rejection, but how long will it take for society to be transgender blind- because that's what we should be striving for- instead of "Frankie, you're a boy? That's FANTASTIC!" something along the lines of "Frankie, you're a boy? That's cool, whatever."

At least, that's my theory. Whatever the reason, I do think it's safe to assume, based on her reaction to the conversation with Max, that Sam figured out that Frankie made the story up and is actually transgender.

Max believes Frankie is a boy because let's be honest, more than 90% of the teens will automatocally think (and gossip about it) someone is a 'boy' if they saw some girl dresses or acts slightly 'masculine' like Frankie. That's how they talk and act. And especially with the restroom incident they will be convinced more than ever, like Max. Frankie could actually be lying to her mom, but we don't have enough material to judge that yet. Personally I believe the story Frankie told Sam. And also I find Frankie to be someone who would come out rather than hide it by making a weird lie.

Sam's rather stressed expression after Max's declaration seemed something like 'should I have stopped her? and made her act more feminine?'. Of course there isn't any need for that and Sam knows she hasn't done anything wrong on that part, but still if her sister is being dead serious saying "she's a boy", imagine what other mean boys/girls might do to her. Sam would definitely feel a bit worried. (Again, I'm not saying she should have advised her to be more feminine or anything)