If pre-marital sex is a sin in the bible then why do so many Christians still have intercourse before marriage?

I recently learnt that in the bible, God forbids sex before marriage. If that's the case, why is it so rampant amongst Christians today?

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It's become so normalised as well. What I don't understand is how Christians can claim to love Christ and God, but then openly commit one of the seven deadly sins. Its like religion and God is just an afterthought to them.
If pre-marital sex is a sin in the bible then why do so many Christians still have intercourse before marriage?
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