Well, the definition of sin is missing the mark and breaking God's moral commandments. Which I see anything about condoms in. I don't believe having masses of kids in poverty is what God ordained in His word. To be fruitful and multiply as given in the Genesis account was a command to populate the earth before disease and slavery of debt took over. We are not under Old Testament law, but under the moral law of CHRIST and are not bound by rules and doctrines of men. If man thinks to impose his laws which are not in the Bible than I see your point, but that is not Christisnity, but legalism. Polymorous relationships were never condoned by God. He gave Adam one wife. Abraham took the maidservant because of his wives lack of faith, but these incidents along with others brought great consequences as God said. Just because you read about the human sin in the Bible doesn't mean God ok'd it, but usually brought judgement on it. The Bible is about God intervening in the lives of sinful men and His redemption of mankind. Etc...
I agree with you about imposing our thinking on different realities. But you have to remember the Bible wasn't written from a western mind set but a Middle East mind set and is a book for all peoples. It is very applicable for all generations in all nations as God intended. Just be careful you don't label things as Christian which have nothing to do with CHRIST or His gospel. Also liars will not inherit the kingdom of heaven, so these who live off lies are NOT Christian. 1 Cirinthians 6