Many Christians today disagree about which parts of Scripture are authoritative, but that does not mean there is no objective truth or that Scripture is hopelessly unclear.
There is always objective truth.
The objective truth may be that there is no god and your scripture may be completely untrue. The objective truth may be that there is a god but your scripture is not the word of that god and is completely untrue. The objective truth may that there is a god, you picked the right one, the scripture is bang on, but people have been interpreting it correctly until now.
You haven't presented very good evidence why your singular version of the truth is more likely to be objectively true. "Because people have consistently believed it for thousands of years" is a pretty bad reason. They have consistently argued for slavery and used the Bible to justify it. Your interpretation of what the Bible says is that it is description not recommendation is a very modern one that fits
Wilberforce used the Bible to justify his crusade against slavery, just as Christian slave-owners used it as a justification for their enslavement of people. That's hardly a win for the core beliefs of Christianity. To me, it shows the progression of social morality sits beyond Christianity and contemporary Christians rejecting what Christians had believed for a very long time because they knew they could no longer justify the objective moral truth that slavery is wrong.
The Bible is unclear about slavery. It condones it. It makes peace with it. It provides workarounds for slave owners and enslaved people.
A book of moral truths that does not hold that slavery is always, indisputably wrong is not a good book of moral truths.
If your book of moral truths does not condemn slavery, I wouldn't pay much attention to what it says about sex before marriage.