Perhaps random was the wrong word. In other movies like "300" it is random. Perhaps pointless better fits. It did nothing for the plot. It was just there for to draw in more people I guess.
Sex is something that you do in private. I don't care to ever see anyone else having sex other than my wife and I.
As for the violence...
I agree the violence was too graphic but I think I have been desensitized to it. Not that it should be an excuse. I suppose if most TV and movie had sex scenes like this I would be desensitized to those as well. Desensitized or not though, it doesn't make it right.