The bible speaks against sleeping with someone that you are not married to. It is a sin. You are defrauding, stealing and taking advantage of the person.

“For this is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God; that no one transgress and wrong his brother in this matter, because the Lord is an avenger in all these things, as we told you beforehand and solemnly warned you.”

‭‭1 Thessalonians‬ ‭4‬:‭4‬-‭6‬

Having sex with a person that you are not married to is spiritual theft and you are being exposed to spiritual nakedness