The Constitution -- a protector or regulator of rights? Or both?

I tried furthering this topic on two other blogs, but since they both seem to have died of late, I'll try to revive it here. In a discussion on same-sex marriage, a friend of mine argued against a constitutional amendment, writing

"The Constitution is not a document that limits freedom. The Bill of Rights tells government what it cannot do. Not what people cannot do. A constitutional amendment banning gay marriage is astoundingly unconstitutional."

He made a similar point in a discussion about an anti-abortion law. Now, I think there's an inkling of truth here, but I have to question whether we get the whole truth.

Indeed, it is correct to say that the Constitution and Bill of Rights protect various freedoms of the people, not only against the government but against other people as well. But isn't the most obvious (and paradoxical) consequence then the fact that precisely as people are guaranteed certain freedoms and rights, they are also denied other freedoms and rights? When you guarantee the right to life, you deny the right to murder. When you guarantee the right to freedom of speech, you deny the right to silence someone simply because they disagree with you. When you guarantee the right to vote, you deny the right to stop someone from voting who is legally registered. I think I've made my point.

Conversely, usually even when an amendment or law is stated in a negative form ("thou shalt not ____"), there is also a positive understanding to it; something is being protected in the process.

The bottom line then, is that I now seriously question the value of any argument against something simply because "it is regulatory" and we have "too much regulation" (BTW I'm guilty of this bad logic too). Rather, we must focus on whether it truly is or is not a good or bad form of regulation (i.e. protection).