What should I do? Should I stand my point at the sake of my son being rejected by his peers? Or should I give in and let him access what I feel is not appropriate for him?
It's true that many of his peers are listening to this music. Tell him that, while you understand his need to be one of the guys, you cannot allow what you deem to be obscene and grossly derogatory messages and lyrics into your home. Explain that the messages in this music are antithetical to your values (e.g. respect for women) and just as you would not allow child pornography or hateful Nazi or KKK literature in your home, you cannot allow these affronts to your family's values, either.
If he pulls the old "You don't know what you're talking about. There's hardly anything wrong with these groups" line, then do your homework and get the lyrics to these groups' songs. Present your educated arguments to him with references in hand. His threat to "just download them from the Internet" should be met with something like "I hope you don't turn this into a cat-and-mouse game, because I would rather loan our computer to someone rather than enter into this power struggle with you."
Your son is resourceful. He'll listen to enough of this music on other kids' CD's to appear like he owns them as well. Of course, you know that, while you may tell him why you disapprove of him listening to this music, you cannot prevent him from listening to it outside your home.
I tell parents to pick their battles and I know many therapists would disagree with my telling you to "walk your talk" in this case. I think letting your kid know what messages you will allow in the sanctity of your home and why -- and sticking to that position -- is a moral lesson for your child, even as he kicks and screams that you are ruining his life. Thanks for writing.