A lot of people who are biblically ignorant equate the recording of events and practices (polygamy, slavery) with the condoning of those events and practices.
The OT is mostly a history book, with later books of prophecy.
Indeed, and they show that they lack the ability to do a proper exegesis of the scriptures and taking the totality of scripture when it comes to any topic.
What I find interesting is those liberals who argue against the scriptures (where God condemns homosexuality) and say that the scriptures are being taken out of context and that God does not condemn same sex marriage or homosexuality.
I then ask them to provide any scripture which condones homosexuality or same sex marriage. They then - typically - say that the Bible is just a dusty old archaic book that is irrelevant. Funny how they quickly go from quoting scripture to bashing the Bible.
Actually the OT is loaded with Prophecy, as well as History.