Sorry to intrude here, but I have a lot of disdain for that phrase. Growing up as a "boy", that phrase would often get thrown my way, usually by my dad. It always hurt me and made me feel like a bad person because I couldn't be what others expected me to be, especially family. Now, it's obvious why this phrase would be hurtful to a transwoman during her upbringing, but I feel like it can equally hurt men. It's diminishing their masculinity and treating them like they are lesser in some way. That's not a nice thing to do. Sure, people can joke around and say it, but it seems like that phrase is often said with disgust and hostility or it's used in the spirit of criticizing/reprimanding. That's why I don't care for it.