There is clearly a subset of the Christian community who feel that they have biblical support for the idea that marriage can only be between one man and one woman. [...] I'm not as familiar with the scriptures as many here, so could somebody talk me through this?
And they are quite correct.
Ge:2:18: And the LORD God said, It is not good that the man should be alone; I will make him an help meet for him.
Ge:2:20: And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him.
Ge:2:22: And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man.
Ge:2:23: And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.
Ge:2:24: Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Here we have the first wife - a woman created by God for man.
I know that there is all that nasty stuff in Leviticus about man not lying with man as he would with a woman.
Indeed - it is pretty clear in the OT and NT as well
Homosexuals (Romans 1:26, 27), The Effeminate (1Corinthians 6:9),
But there is also gobs of stuff about how many sheckles you owe your neighbors if your goat eats their crops. And what kinds of clothes you are allowed to wear, and what you are allowed to eat. The abominations are pretty extensive.
Indeed, but none is more or less important than any other.
The BIG TEN Commandments say nothing about marriage. And aren't those the BIG rules?
No. There are at least 613 Commandments. "The 10 Commandments" are not seen as the final word at all.
Jesus seemed to have opinions about divorce, but as a single man himself, didn't seem to be such a big fan of marriage.
Neither did he diss it.
Didn't he even encourage folks (men, I guess) to leave their families and follow him? Can anyone help me out on the Jesus end of it?
The great love is for your wife and family. The greatest love is for God - it transcends everything.
But god didn't seem to have a problem directing some of his favorite humans to engage in behavior that surely falls outside of the traditional marriage model. I mean, god told Abraham to have sex with Hagar, and he didn't seem to have a problem when Lot offered up his virgin daughter to be raped by the mobs outside the door. And when Lot had sex with his daughters, it was their fault, not his? And he was the most righteous guy in town? And he got a hard on for his little girls? That sure is setting the bar pretty low, isn't it?
No. Women are property and, since Eve's idiocy, are not seen as capable of behaving properly without a man around to guide them - see Lot's daughters: once male influence was removed, they turned into harlots.
So where are all of these biblical rules prescribing marriage?
See Genesis above, although I'm not too sure what you mean by "prescribing marriage."
And how do we know they trump all the other stuff in the scriptures?
In fact, a lot in the Bible suggests that marriage, for a man, is a secondary goal. Men are told that they would be better of as eunuchs; as you point out, they should leave their wife if there is a conflict between her and God. However, Paul (in Timothy) suggests a Bishop has to be married for the demonstrable responsibility and as an example.
Or perhaps, more importantly, why do a subset of monotheists believe that their god wants them to make it a priority to make a fuss about family structures. I think it is pretty clear that the scriptures ARE NOT clear on this topic. So what is the real motivation?
I do not think that you have even started to make the case that the Bible is unclear. Indeed, you say, "I'm not as familiar with the scriptures as many here,"
Judea of 30AD is not the US of the 21st century. Homosexuals have traditionally been seen as filthy perverts, so disgusting that death is the only answer. Think of today's paedophiles.
If people do accept the Bible, then it is clear to them. Democracy in the US requires that the people be listened to - even if they are wrong. The Bible is an irrelevant set of folk tales. The important thing is "What the Bible says does not matter." Those who think it does matter are grossly ignorant - but then that is Christianity/Islam/Rastafarianism, etc. - and religion is the manifestation of ignorance.