I realize that most everything I'm about to say will be panned but figured I'd throw in my two cents even though it will be scoffed at by some I'm sure.
I fully believe that God created the woman for man as his helped, like what is described in Adam and Eve in Genesis here:
"18 The Lord God said, “It is not good for the man to be alone. I will make a helper suitable for him.”
19 Now the Lord God had formed out of the ground all the wild animals and all the birds in the sky. He brought them to the man to see what he would name them; and whatever the man called each living creature, that was its name. 20 So the man gave names to all the livestock, the birds in the sky and all the wild animals.
But for Adam no suitable helper was found. 21 So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs and then closed up the place with flesh. 22 Then the Lord God made a woman from the rib he had taken out of the man, and he brought her to the man.
23 The man said,
“This is now bone of my bones
and flesh of my flesh;
she shall be called ‘woman,’
for she was taken out of man.”
24 That is why a man leaves his father and mother and is united to his wife, and they become one flesh." (I added the bold to the text)
If you at all subscribe to the belief that God is real and God created the human race, you can see where the divide on the subject comes from. Yes, I would agree that marriage predates Abraham, considering these were the first 2 people created according to the Bible. What God considered a "suitable helper" for Adam was a woman. I imagine the land had to be taken care of among other things and obviously for having children, and a woman was created to fill that role.
Numerous times throughout the Bible, it is very clear about God being unhappy or even upset about sexual immoral acts, including homosexuality. Do I condemn a gay person for their choices? The answer is, no more than I condemn straight people for their sexually immoral acts. I've been with a lot of women in the past and it was something I was personally remorseful for and I needed forgiveness for those sins just like any sinner would. Gay people are in need of forgiveness no more than anyone else.
As far as marriage, in my personal belief, gay people cannot be "married" as defined by God's definition of holy matrimony in the Bible. That is reserved for a man and a woman. Gay people should be allowed by the government if that is decided for civil unions for tax purposes and all that though I don't agree with that lifestyle one bit.
I also do not believe they should adopt. Granted, there are millions of unfit parents out there, I think there are a number of reasons why the birth parents or the legitimate family of the child should raise them but that's a long long conversation in itself