Your point that the Seventh Commandment is designed to protect community is well taken. As I think I said quite a while ago, I suspect, following Huston Smith, that all ten of the Ten Commandments point to those acts that, if committed, would lead to a breakdown of society. It is one thing to lie; it is another to do so in court. It is one thing to kill, it is another to murder, etc. People have to trust one another and the system if order is to prevail.
I agree regarding monogamy as well. While we uphold the ideal of life-long monogamy, as a society we practice serial monogamy. We marry for a while, then divorce, and then marry again. This provides us with legal cover for the biological imperative to have multiple partners.
You mention two ways people deal with this commandment: they either ignore it in favor of multiple partners or take it to mean that we ought to damn sex itself. There is a third alternative, God’s alternative you might say if you take the Bible as the literal Word of God, and that is polygamy. Taking multiple wives, and being bound to them legally, economically, morally, etc., is the way God handles the problem. Still sexist, of course, but God is hardly a feminist. Nowhere does the Hebrew Bible condemn polygamy. On the contrary, it regulates it.
Exodus 21:10 says, “If a man takes another wife he shall not diminish the food, clothing, or marital rights of the first wife.” Deuteronomy 21:15-17 says that the first-born son of a polygamist family has the right of inheritance even if his father who favors another wife and her children despises his mother. And Deuteronomy 17:17 warns kings against taking too many wives.
And then there is the practice of Leverite marriage where the brother of a man who dies childless is obligated to marry his sister-in-law, even if he himself is already married, and have children with her, assuming the sister-in-law agrees (Deuteronomy 25: 5-10).
Of course we can argue there were valid socio-economic and culture reasons for all of this, but that doesn’t negate the fact that the Bible does not ban polygamy. In fact, polygamy isn’t banned in Judaism until the Herem (ban) of Rabbenu Gershom in the 11th century and that only applied to Ashkenazi or European Jews. Sephardi and Mizrachi Jews (Jews from Portugal, Spain, Arab countries, and Iran) never banned polygamy, though most dropped the practice as they emigrated to countries that outlawed it. Modern Israel limits the practice but makes room for polygamous families immigrating from countries where polygamy is legal.
My point is that this practice has a long history of legality. In the United States polygamy is in the news a lot lately, and people are officially offended by it. But the real offense, it seems to me, is child marriage and rape. To the extent that these are happening under the cover of polygamy is a crime, but consenting adults who wish to practice plural marriage—they may be on to something.
I want to come back to the holiness of sex later, but let me stop here and invite your comments.