I don't know about this kiss - when watching the scene I see something sexual shown by 2 complete strangers to each other (Spears trying to jump on "this" train now?) in order to create media attention but when I see tATu I see feelings/affection - but that's just me.
Edit: in a post above the "tongue-action" was mentioned - to me the little pecks on the lips after the kiss from tATu tell more than this sticking the tongue to the tonsils by 2 complete strangers.
Besides these two persons at the VMA - one of them is a married woman with 2 children, at least now clearly straight and the other one was known as Ms. Go-into-marriage-as-a-virgin *cough* and quite clearly straight as an arrow - that's a show, there's not the possibility that they're really into this direction while with tATu it's at least to me clear that there is/was more going on between them.
There's something different between showing something like a kiss from persons who are clearly acting and persons where there is the possibility that they're not.
Don't get me wrong if this VMA-show has positive effects than excellent (!), but to me it's not a statement for whatever, it's only a show "Woooo, 2 hot chicks sticking their tongues into each other's mouths and by the way we're so married and christian - so don't be afraid for your kids everyone knows we're straight".
