Taylor Swift - So It Goes... Lyrics Meaning

It's about lesbian sex. 'Wear you like a necklace' that's pretty darn near explicit. Also the secrecy of the relationship. 'Gold cage, hostage to my feelings' implies that she's stuck in a cage and can't express here true feelings. And 'I'm not a bad girl, but I do bad things' why is sex bad for an adult? unless it's forbidden in some way.