I think I do see your point
I've snipped your OP a bit, but don't worry, I read it all. So, because adultery really only concerns God and his children, this is what is meant by adultery. Meaning, of course, that marriage between a man and a woman is irrelevant on the topic of adultery. So, this makes the situation rather simple, yet more complicated.
Because if we say that the only adultery that can be committed is between God and his people, where does that leave a married couple? In other words, how should a married Christian couple live their lives? Does it mean that if one partner cheats on another, that this still isn't cause for divorce? How does this, if at all, apply to earthly marriage of any kind?
Does this mean anything directly or indirectly from homosexual Christians, who are married to God and in covenant with him, but happen to love someone of the same sex?
Let me first say about your first point, that God also expects us to understand the bible in our own lives, but if we approach trying to understand the bible only that way, we will indeed miss the real truth. So while the bible and this whole existance is in fact about God and his marriage to his people, it is also for us so that we can begin to understand what he is doing for his people. Some similarities like passion, sacrifice, friendship, that we are suppose to find in out marriages, is what God finds in his chosen people.
Also, note that God recended the law to allow divorcement, so he is never going to divorce again, which means we can not divorce anymore even for the cause of fornication or adultry.
Now lets expand on this a little bit. Suppose when we commit spiritual adultry with God, like go after other gods, or worship a dead object like gold or whatever. Every time we do this according to this thread God could divorce his people. Well glady he does not, he forgives his people. Throughout the OT here was very forgiving to Israel often times giving them a chance to come back to God, sometimes they would and often times they wouldn't.
If you remember in the OT, Israel was jealous of their neighboring countries because they had a king to rule over them. God knew their heart and said, since you are not satisfied with me, I will make you a king for you heardned hearts. And so God raised up good and bad kings to rule over Israel.
Anyway we can see that the 1st bride had free access to God, what a gift to just walk away from.
I digress a little bit to say this, in our marriages on the earth we do not generally have the mercy God had for thousands of years with Israel. Especially in todays world, we will get divorced for just about anything. See what I am getting at? We have taken one line out of the entire bible (which we mis-interpret) and have given rise to more divorces than ever before since the church began. So you have to distinguish between the church and the bible. They are not the same.
Gay marriage? Well, the new testament refers to this as men and men or women and women working that which is unseemly, changing the natural affection of man and woman, burned in their lust one for another. So obviously know one knows who God will choose to take to heaven. That is God's business, but it is clear that homosexuality is just a deviation from the natural use of mankind. Any time we are burned in lust for one another not under the lawful institution of God, and have sex it is sin, whether we are gay or straight.
Remeber marriage is not something that is governed under a church, it is just whatever the law of the land happens to be. So if two are married it is then justified, as long as they have not been married before. If they have been married before than they should just treat this as their first marriage, and realize they sinned into this marriage. Though if two people know it is sin, and then decide to get married, that is just evidence they are "probably" not a child of God.
In terms of two gays getting married and staying married until they die or Jesus comes back, well this falls under the category of trying to know who is going to heaven. I believe gay people can in their heart serve God, and also realize what they are doing is against nature, and be torn inside for their whole lives. Of course there are gays who could care less about what God thinks, just as there are straight people who could care less. The bottom line is if you are a "christian" and want to follow the only words we have from God, which is the bible, than we have to conclude that gay relationships are probably not what God intended for marriage. I don't condemn them, as God can save whomever he will.
To each his own and all of that.
