the closet is glass

is used to tell someone that they're borderline homosexual even though they claim otherwise, which is telling them that the closet they're hiding in is transparent, everyone can see them clearly in that closet.

"Dude, have you heard Lady Gaga's song 'Boys Boys Boys'? I'm not gay but it's just sooooo good! She's such a queen."
"Man, the closet is glass .You can just say it at this point."