Here's how I see it - you aren't at all to blame. Were you all sober, you wouldn't be to blame at all anyway, but since the girls were drunk, you had a certain responsibility, which I think you fulfilled by encouraging them to wait. So that's that out of the way.
The attacker is somewhat to blame, and so are the girls. They ignored your advice and furthermore ridiculed you for it, which is not something I feel is within the bounds of excusability, drunk or not. If you hadn't mentioned that they may be assaulted, and there was no reasonable expectation, it would have been entirely the attacker's fault, but seeing as you had warned them and established that there was a chance they would be attacked, they went aware of that chance. In the end, it's like standing at the edge of a cliff, falling to your death and trying to attribute blame. Of course your death is entirely because of gravity. But you also shouldn't have been standing on the edge, you idiot. So it's entirely the girls' fault, entirely the attacker's fault, and not at all your fault.
And if your friends blame you for they're all dicks. Without exception.