This is one of the main reasons I hate 'lolol Justin Beiber is a gurl' jokes.
People may not literally make the trans* joke often, but the implications there are more than enough. He doesn't look or sound manly enough...ergo, he is not a boy; he must be a girl. Better yet, a girl pretending to be a boy!
It is not okay to use 'you're a girl' as an insult. That's mysogyny.
It is not okay to attack someone's gender identity. That's transphobic. EVEN IF THE PERSON IS, OR PRESUMABLY IS, CIS.
Because if a cis person's gender identity isn't safe from attack just because some people don't like his music, how is there any hope of
my gender identity ever being safe.
And of course I'm sure there are plenty of people who parrot this joke without really thinking about it. Because it's the internet, and it's a meme. But ffs that's not an excuse. Think about what you're saying, don't just repeat it because other people are laughing.
Which reminds me of the year my high school German class became obsessed with the mudkips meme...without actually knowing the story behind the phrase. Everybody using it got in trouble when the teacher actually looked it up. They had no idea the origin of the meme was sexual, and were perpetually confused. It was honestly pretty hilarious.