Continuing from part one...
2. How can you say marriage is about procreation when not all marriages involve procreation. Does that mean that those who cannot conceive cannot marry? What about Mary and Joseph?
That many families exist without a father and a mother doesn't negate that a family, normally, includes a father and a mother. That some people's eyes and ears do not see or hear, does not call into question the truth of saying that eyes are meant for seeing, ears are meant for hearing.
Likewise, that when a man and a woman are drawn together by natural attraction, and enter into marriage, yet they cannot conceive--and maybe they know it, maybe they don't--doesn't negate the origin and natural reality of where marriage came from and what essential purpose it serves for the human race, to the moment I am finishing this sentence.
3. You say marriage arises out of human nature--but doesn't homosexual attraction also arise naturally? So why doesn't that justify same-sex marriage?
I'll answer the question, but it's not really a question that challenges defenders of marriage-as-heterosexual; it actually is a problem for "gay marriage" advocates. If, as you say, homosexual attraction is nothing new, then how is it that only in the late 20th century, in one part of the world, has it suddenly become "obvious" that marriage has always been arbitrarily, and too tightly, restricted to heterosexuals? Blaming the Bible for stigmatizing homosexual behavior only explains what cultures influenced by Biblical prohibitions have done. How does one explain why all other cultures, whom the Bible has only recently influenced, yet have never had such a thing as same-sex marriage?
Once again, we might look to the example--so frequently cited by the other side--of the supposedly gay-friendly ancient Greeks and Romans. The usual narrative goes like this: they didn't harbor the narrowness that Christians are blamed for introducing into western civilization, and which is only now being overcome. So then, how explain they still didn't develop same-sex marriage?
4. What is the role of government in this matter?
This is an important and interesting question, and to a great degree, it depends on your understanding of the role of government.
As an American, with conservative and some libertarian views, I hold that our government is one whose powers are neither arbitrary nor unconstrained. I didn't invent that view, of course, but there are some who seem to think this a novelty. They view government more expansively and practically: if something needs to be done, it might be just as well to have government do it.
I asked the question in the comments in another thread, who gave the city of Washington, D.C., the authority to create "same sex marriage"? The other commenter seemed to be puzzled--as if to say, something needs to be done, the city is as good a way to do it as any other.
A lot of wish to live in a society in which the powers of government are tightly restrained. The Constitution doesn't specify our rights; it presupposes them, and specifies what powers government may undertake in our name. Only to the extent specified, may government restrain or step upon our rights. We retain our natural rights, even if unenumerated in the Constitution. Interestingly--although I fundamentally disagree with it--the line of Supreme Court decisions leading to Roe v. Wade included this argument, in explicating an unenumerated "privacy" right in the Constitution.