I think we need to look at the following
1 Corinthians 6:16-20
16 Or do you not know that he who is joined to a harlot is one body with her? For “the two,” He says, “shall become one flesh.” 17 But he who is joined to the Lord is one spirit with Him.
18 Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body. 19 Or do you not know that your body is the temple of the Holy Spirit who is in you, whom you have from God, and you are not your own? 20 For you were bought at a price; therefore glorify God in your body and in your spirit, which are God’s.
When God created Adam had Eve they became one flesh as a result of having sex.
Genesis 2:24
24 Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh.
Paul quotes this in v16.
When a person becomes a believer they are united with Jesus.
When a believer commits adultery or imorrality they are basically doing something with the body that is joined with an adulteras relationship yet the person is also joined with Jesus.
We need to look at what Paul says a few verses previous
1 Corinthians 6:12-13
Glorify God in Body and Spirit
12 All things are lawful for me, but all things are not helpful. All things are lawful for me, but I will not be brought under the power of any. 13 Foods for the stomach and the stomach for foods, but God will destroy both it and them. Now the body is not for sexual immorality but for the Lord, and the Lord for the body.
When two people become one the Lord is the third person in this relationship.
The following verse is often quoted at marriage ceremony
Ecclesiastes 4:12
12 A person standing alone can be attacked and defeated, but two can stand back-to-back and conquer. Three are even better, for a triple-braided cord is not easily broken.
When two people get married they do so before God and promise to keep a lifelong commitment to each other with God as the centre of the marriage.
If we look at 1 Corinthians chapter 7 we see Paul talks about marriage and keeping the marriage sacred as it was always intended to.
Sexual sin has serious consequences.
I know.
I have never commited adultery yet even as an immature Christian I commited fornication, having sex. I was never taught about it.
Yet every time I did the guilt and shame was massive and oppressive.
I cannot make any excuse for what I did but I did so as a result seeking some kind of love as a result of my upbringing.
Lack of love, was sexually abused and a lot more.
Not unusual for a person like to seek sex for reassurance.
Most of my family have committed adultery during my upbringing.
YET
As a result I had a lot of baggage that I carried as result.
In a sense when we have sex we create a soul tie.
I look at this way.
If we have sex with someone then it's like a ruler. The ruler is 30cm long and complete.
Yet when you break the relationship and have sex with someone else the ruler is now broken.
When it is then we have jagged edges. This causes pain, guilt.
Do it again and again we have more jagged edges.
Praise God he healed me of the guilt.
So for me it's a sin against the body because we have body, spirit and soul.
We are united with our partner and Jesus as it was meant to be.
Bring someone else in to it is like asking Jesus to bless and that he will not do or bless.
Sex is just flesh on flesh but it body, mind, spirit and soul.
I hope I make some sense.