Sorry, but I DO think it is a big deal! Do you let this 8 year old wear makeup or suggestive clothing? Do you let her do other 'adult 'activities? of course you don't! Singing about being a love salve and being whipped for fun is just plain gross in a culture like ours that totally sexualizes little girls. Girls get a LOT of their future self-image from things like clothing, music, etc. Don't set this cute, smart 8 year old to have a negative, overly sexed up self-image at such a young age!
My son is nine and loves music. He used to make little playlists on my iPod, so we got him his own last year. He loves to listen to classic rock like the Beatles, etc. but also songs from the radio. HOWEVER- there are a lot of songs I do not allow him to download for the reasons you just described.
Now, I am not a fan of 'fake' Disney manufactured type teen singers like Jonas Brothers or Hannah Montana. I like it that my son listens to real music by real bands. But we have vetoed many downloads just because the song is too adult.
I would have a serious talk with the girl's mother- it is just gross to have a little kid singing about bondage or whatever. I remember last year Flo Rida was on AI and my son heard his song about 'going down, going down' - which is OBVIOUSLY about giving head- and asked my fiancee (soon to be his stepdad) to download it for him. My fiancee double checked which song is was and quickly vetoed it.
I explained by saying that even though it was a catchy song and my son liked how it sounded, that some of the words also meant other, more grown-up things that were just inappropriate for him to listen to. He was a little miffed, but we downloaded something different.
You are being responsible here- I would definitely not let this issue drop with the mom. You don't want your cute 8 year old sounding like a skank!!!