Edit: I have no idea why the responses below have been taken over by pedants lacking in reading comprehension. Lordosis is the name for the behavior, the reflex, the posture, all the things. We’re taking about presenting the vagina for copulation here. That is what the songs are talking about. It’s not literal mating, but it’s sexually exciting. These behaviors are deeply embedded in the primitive levels of our brain. It’s even hypothesized that this is why we have butts at all.
The part/whole phenomenon is called synecdoche and is extremely common in all forms of communication.