Well, if you guys can bear with me for a second, I know where the disconnect is.
The people against the song believe that it's presumptuous. Women have been assaulted before after claims of "we both want this," or "I know you want to have sex with me."
People who aren't against the song are thinking of it in a situation where Thicke and co. is correct - i.e. the proper context. There's no way where you can actually know this, but something I read the other day sort of clicked with this and I realize that this is the same grey area that GAF will never get over.
What I read was a woman complaining about guys asking if they can kiss her, because she wants spontaneity and the asking puts her off. Obviously, in any sort of sexual situation, you do want consent from both parties. At the same time (and previous female posters here have noted this), you may also want people who just read the situation properly. As an example, we all know that spousal rape is a real thing. But in the proper context, with two people who are aware of each other, they may not necessarily ask each other and confirm (as is the advice for random hookups).
Or even with an SO. But people may read a situation completely wrong and end up doing something regrettable. When it comes to a situation where you're dealing with strangers, there are women who will want one sort of thing and others that may not. The only way to make sure that you don't fuck up in such a situation is either to make entirely safe moves (which may not be appealing to some women) or just learn proper social context. But the latter can't necessarily be taught or quantified, so the only way to figure it out is to interact with people.
So the situation that the song "Blurred Lines" describes is one that is presumptuous, but can work in certain situations. It's not a song about rape or sexual assault, but guys who do the same thing in real life (particularly socially inept guys or guys who are extremely narcissistic) can end up in sexual assault territory very quickly. The song title is characteristic of the situation - it is very much a blurred line, and there's nothing I can say here that will help anyone make heads or tails of it.
It's just what happens if you're going to flirt at a club.