Where did this notion that Britney is homophobic come from?

You can call her a lot of things, but homophobic?
If anyone's homophobic, it's Nicki. She has MULTIPLE verses where she denounces gays.
You can suck a dick
You can suck on a ballsack
No, no, I don't endorse that
P-p-p-p-p-pause that
A-a-a-a-a-abort that
The queen of rap
Slayin' with queen B
If you ain't on the team
You playin' for Team D
