I've already given my reasons. there is an overwhelming amount of evidence to support why she thought the way she did. As I said, she has been living with this her entire life. To people that don't deal with this sort of thing, it is presumptuous and can come across as arrogant and judgmental. 

To Quinn, she is dating this because maybe, just MAYBE she is excited to see that there is a possibility she isn't alone in this school. Yeah, she jumped the gun and is running away with an idea as if if were fact. But what you think about it OOC doesn't matter. She IS flawed, but this is something she knows about more extensively than most of the world. 

you guys keep making this about me when it isn't, you especially Leonardo. This isn't about me at all. It is about Quinn trying to make a connection. Not to show that she knows everything and she can guess your gender by using logic. She has her own fears and insecurities about the surgeries and hormone therapy, and here is someone that could very, very likely have gone through it. She is jumping to a conclusion wholeheartedly believing it is true. Let her and then let her deal with the consequences. 
 Again, had you lot just let the RP play out, you would have learned her intentions, reasons, and why she'd even speak up about it.