The Bible has been used to justify everything you can think of. All texts are social constructs. It is easy to erroneously extrapolate this to the whole Bible. Biblical illiteracy is rampant. Most people have never read the Bible cover to cover, or even carefully read key biblical books like John or Romans.
Church history records ugly disagreements over the Bible's meaning.
To many, this links the whole idea of biblical interpretation to ugly controversy. Relativism undermines confidence in objective meaning. I believe this is one of the most prevalent and insidious objections to Christianity today, subtly eroding the Bible's authority for both non-Christians and Christians. Only a cruel, sadistic God would purposefully record his message in such an obscure way that we could not be reasonably certain we understand it. By using language that is sufficiently clear.