ThePerfectFather - I am struggling to understand what your point in this is. Earlier you said you only have a (limited) respect for people who believe their text 100% literally. But then you said they are morons for believing it that way?
So, you have decided there is only one way to "do" religion, and it is stupid?
But then you say I am wrong for believing in a way which doesn't make me a moron, just because it doesn't match up with the only way that you consider religion should be allowed to work.
I think your definition of how religion works, and how Christians are "allowed" to interpret the Bible is very narrow, and one which the vast majority of Christians down the ages would disagree with. There has never been a time when all Christians believed the same things, or interpreted the Bible in exactly the same way. There have been debates and arguments and splits in the church almost from day 1, many of them over issues like how the Bible can be read.
The Bible doesn't say "God changes his mind about stuff", but if you believe (as I do) that the Bible was written by humans, with an imperfect understanding of what God wanted, then it is perfectly possible to read the Bible and say "People at that time thought that God wanted X, but this was affected by their experience and culture. We have an improved understanding of some things from what they knew, and can reconsider whether this text can be looked at differently"
So - sex before marriage. In Bible times, if you had sex you got pregnant. A single woman had no source of income except maybe through prostitution, and would not have been able to support a child. She would also be very unlikely to then ever find a husband who would take on an "immoral" woman and her illegitimate child. So for a man to have sex with an unmarried woman would have been a sin, because it condemned her to a terrible life. Equally for him to have sex with someone else's wife would have been a sin, as there would be unfaithfulness to the husband, plus the chance of leaving another man to raise his child. The sin was in the harm, and the harm (then and there) was always serious, so sex outside of marriage was sinful.
That is no longer the situation - women and children have rights, and a single mother can support herself and her children and provide a good life for them. There is no harm, so there is no sin.
The people who wrote the Bible could not imagine a world very different from the one they knew, and the way things had always been, so they believed the message God gave them would apply to all people and places, rather than adding explanations and caveats.