When discussing/defending the Bible, it's hard especially when we bring in possible past translations. If you want to go down that road, then "sin" isn't actually an act or crime that separates you from God as it was translated from the Greek word, "hamartia" which means "missing the mark". Then you'd have to start looking into other books and scriptures and gospels and books that aren't even in the Bible, but are definitely Christian literature. Then discerning which are based off oral traditions, or possible re-writes of older versions or early translations. THEN you there's the whole no reference-able authors to the books of the Bible.
It's all very messy, and I'd rather simplify things for myself by looking to myself, to my community and society, and current writings for truths about morality and looking to life to give me valuable life lessons. I personally think the Bible gets talked up more than it should. I kind of view it like any other way of learning something; if you want to learn the truth about the subject, get up-to-date information. If I want to understand modern English mechanics, I'm not going to look at Shakespear, then pick and choose what I feel is best used and discern that as the best source available for such knowledge.
Same with morality and marriage. I'm not going to look to the Bible for those truths when I have up to date, modern sources that actually relate and come from a society and culture with certain pressures and environments. So, I think, when dealing with marriage, we have deeper truths that don't rely on gender/sex role assignments, but rather deal with significance of the individual, which didn't make sense to do 2000 years ago in the Judeo-Christian cultures.
Sorry if I can't offer any useful answers for you, but this really is a wonderful thread so far, haha!