>>That’s Vatican law. Not Biblical law.1 Cor 6:15-16
15
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! 16 Do you not know that he who unites himself with a prostitute is one with her in body? F
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Ex 20:14
14
"You shall not commit adultery.
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Prov 6:32-33
32
But a man who commits adultery lacks judgment; whoever does so destroys himself.
33
Blows and disgrace are his lot, and his shame will never be wiped away;
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Matt 19:4-64
"Haven't you read," he replied, "that at the beginning the Creator 'made them male and female,'a 5 and said, 'For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh'?b 6 So they are no longer two, but one. Therefore what God has joined together, let man not separate."
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You seem to confuse the term “adultery” (marital infidelity) with premarital sex.
The former is indeed a sin. The latter isn’t.