I don't find this song to be about Usher getting it in the ass from a girl. Basically he's switching roles with the girl (not penetration-wise). She'll do the stuff he normally does before sex and act the way he does during sex. Instead of being on the top like he normally is (apparently) she is going to be on top and he'll be on the bottom. Hence - Trading Places. Just listen to the song while having sex and you'll be perfectly fine.
I don't find this song to be about Usher getting it in the ass from a girl. Basically he's switching roles with the girl (not penetration-wise). She'll do the stuff he normally does before sex and act the way he does during sex. Instead of being on the top like he normally is (apparently) she is going to be on top and he'll be on the bottom. Hence - Trading Places. Just listen to the song while having sex and you'll be perfectly fine.