"Is that not what Christians argue, as well?"
Pretty sure, not. A sampling of this forum, say any given thread where Dannyboy and I spar, should show otherwise. I'd say we are representative.
"I thought that that was the point of free will. God let's us make our own choices, even though those choices may be tough."
Sure God let's us make choices, but that doesn't mean they are to be unbounded. Some choices involve difficult moral calculation. That's different than believing there isn't a basis for moral calculations at all.
"If we do not, then society will be weaker."
So?
"It is in our best interests to stop doing things to hurt ourselves."
So?
If the subject is torture, it seems like you'd be arguing, or at least welcome the argument, that the torturing of a few is definitely in the self-interest of the many, especially when it is nearly certain the subject of the torture wants to kill lots of people and probably has information on plans to do it.
Your argument here seems to be laying a pretty solid groundwork for undermining the moral authority you invoke in your opening post.
"There is an element of self-interest in my moral prescriptivism."
Mine, too, actually. The difference is that I believe that moral law is not based on human opinion, which can shift and change, but upon the design by the one who made humans in the first place. I believe that this morality has our interest in mind, not just because of the notion of fear and punishment (as you frequently assert) but because I believe that there is a surer path to happiness and contentment, individually and socially, for those who adhere to the Maker's design.
"There is. Society determines the standard."
And society is composed of humans who constantly shift their opinions, especially when it is composed of humans who feel competent to engineer their own standards. Saying 'society determines the standard' merely moves the question back a step.
"I seem to recall that you have argued that Christians are in general agreement about the more important aspects of morality. "
All humans, actually. Because I believe we have a common maker.
"The only difference is that you base your standard on the authority of an imagined deity--a superbeing that controls everything."
It is of course begging the question to dismiss it as an 'imagined deity.' It makes obvious good sense to conform to the standards set by one who is in proper authority over you. In my book, the one who made us is in the best possible place to tell us what is good, best, and proper, and the converse.
I'm not sure how you can square your argument here about us believing in a 'superbeing that controls everything' with your earlier statement that we God gives free will. Do Christians believe God controls everything or that he gives us free will? Settle this for me, Cop.

"God is benevolent in the sense that he has our welfare at heart."
Ok, I see you grasp that much.
"You base your morality on opinion just as much as I do."
To the best of my ability, I base it on God's opinion. That seems proper if he is the Author of all.
"We do not fundamentally disagree on mathematics. We do disagree on the existence of gods."
This is a red herring. Whether or not we agree on the existence of gods is irrelevant. The mathematics was just an illustration. The raw fact is that you claim there is no moral standard. Any statement of judgment about anything involves some kind of standard. The question is what they reduce to.
Dannyboy's favorite example is ice cream. He thinks that saying one thing is morally right is about the same as preferring chocolate ice cream over strawberry. And on his terms, I'd say he is right. I, however, have trouble putting questions like 'torture' and a horde of other issues as like preferring one flavor over another. I especially have a problem with people making an argument on the assumption that we should all, for some bizarre reason, have the same preference, if there isn't in fact a standard that transcends us all.
Its another example of cake and eating it too. You want the moral high ground but don't want to face the implications. You make moral pronouncements while rejecting the existence of the kind of basis that would make those pronouncements meaningful.
"Again, there is a difference, but we tend to arrive at the same place MOST of the time. Christians tend to be humanists, too."
Christians are the founders of humanism. The father of humanism was Erasmus. Have you ever read his works? He was devoutly Christian. So, I think the equation should actually be quite the other way around- atheists tend to be humanists, too. However, modern humanists have detached from the mindset the principles that Erasmus had in mind when he 'founded' it. Modern humanism is just a bastardized form of an originally Christian worldview.
Modern humanism lives off of the inherited capital of Christian forerunners, but on their own terms- ie, spurning the very well spring of the principles you hold dear. Erasmus never dreamed of promoting human rights and dignity apart from our position relative to God. In this way, he was really able to promote human rights and dignity because he was appealing to our common nature as being made in the image of God, a fact which cannot be abolished by the changing whims of society.
Cop, your humanism is just Christianity warmed over. Separated from the original source, humanism has changed over and over again as it was forced to come to grips with reality. Three Humanist Manifestos in under a hundred years and they still don't got it. One Bible has stood well for more than 2,000 years. And it will stand for another 2,000 years, after 'society' and the humanists have penned another thousand manifestos.