You keep talking about sex as the physical act, but ignore the spiritual act of sex. Becoming one, having sex benefits the body AND soul.
Yes there are benefits spiritually
within a covenant marriage but there are also spiritual dangers outside that covenantal marriage. Paul warns us of the spiritual danger’s.
“Do you not know that your bodies are members of Christ?
Shall I then take the members of Christ and make them members of a prostitute? Never! Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, “The two will become one flesh.” But he who is joined to the Lord becomes one spirit with him. Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.”
1 Corinthians 6:15-18 ESV
Alluding to ungodly soul ties which was a spiritual bond meant for a covenant marriage not outside marriage. These spiritual bonds were meant to be a good thing. But if done in an ungodly manner Satan uses it and twists it. Seen it happen, I’ve experienced it even.