Scripture: Genesis 2:24-25
That is why a man leaves his father and mother and is united to his wife, and they become one flesh.
25 Adam and his wife were both naked, and they felt no shame.
In Genesis 2, the Scripture says that when a man and woman get married, they become one flesh. In 1st Corinthians 6:16, it says that when a man has sex with a prostitute, he becomes one with her in flesh. Well, what does that mean? What does it mean to become one flesh with someone?
First, let's think about this very literally. What does a man do with his wife that he can also do with a prostitute? Have sex, right? When a man and a woman have sex, whether they're married or not, a part of the man's body enters the woman's body. They're joined. He is inside her. They are one flesh for as long as penetration lasts.
When Paul tells us not to become one flesh with prostitutes, he's telling us not to defile our bodies because our bodies, being inhabited by God's Holy Spirit, being members of Christ's Body as part of the body of believers, are holy themselves.
So, we need to exercise discretion. We only want our bodies to be one in flesh with someone who is also a member of Christ's Body, who is also holy themselves. That's part of why Paul tells us not to be unequally yoked as well. As holy beings, even in body - not just in spirit - we need to be joining ourselves to, or becoming one flesh, only with a partner who is also holy. God has created us as whole beings - spirit, mind, and body - and He cares about our whole being, not just our spirits. God is very concerned with how we treat, and what we do with, our bodies.
It's important to remember that Adam and Eve felt no shame of being naked with each other because they were both holy. When we should feel shame is when are naked with someone who is not holy. And part of this being holy or not being holy is not simply the idea of uniting yourself to other Christians in sex but uniting yourself in the way that God has commanded us to. That means following the example of Adam and Eve in this first part of Genesis - one man and one woman being solely committed to each other.
That is the model that God has given us for marriage and for sex, and it's what Paul and the rest of Scripture warn us to maintain.