Why is it so bad for a Christian to sin sexually?


Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, “The two will become one flesh.”  But whoever is united with the Lord is one with him in spirit.
Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body.  Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your bodies. 1 Corinthians 6:15-20
            If a person is a believer, he has the Holy Spirit living inside him.  That means his body is a temple for the Holy Spirit, where the Holy Spirit dwells.  If that Christian engages in sexual immorality, he drags the Holy Spirit into those situations, putting God in bed with that prostitute, uniting God with that homosexual lover or forcing him to watch porn.
            Paul gave Christians a unique way to see their sexual immorality, helping them understand they are not alone, but they’re putting God into that sin too.