We know God (here) and He wants our obedience.
Saint Paul says in I Corinthians Chapter 7, 1 - 4.
Yes, it is a good thing for a man not to touch a woman;
yet to avoid immorality every man should have his own wife and
every woman her own husband.
The husband must give to his wife what she has a right to expect,
and so too the wife to her husband.
The wife does not have authority over her own body,
but the husband does; and in the same way,
the husband does not have authority over his own body,
but the wife does."
Let's read the text. Saint Paul says that we should have just one wife (for man) and husband (for woman), and our bodies are just for our spouse.
We should NOT have sex before the marriage because the sex has 2 purpose: union and reproduction.
The dating is a not committed relationship, while sex is a act that brings people together forever.
The union (in marriage) is forever, one man and his wife are together until the death.
Saint Mark says in Mark Chapter 10, 1 - 12.
And again crowds gathered round him,
and again he taught them, as his custom was.
Some Pharisees approached him and asked,
'Is it lawful for a man to divorce his wife?'
They were putting him to the test.
He answered them, 'What did Moses command you?'
They replied, 'Moses allowed us to draw up a writ of dismissal in cases of divorce.'
Then Jesus said to them, 'It was because you were so hard hearted that he wrote this commandment for you. But from the beginning of creation he made them male and female. This is why a man leaves his father and mother, and the two become one flesh. They are no longer two, therefore, but one flesh. So then, what God has united, human beings must not divide.'
Back in the house the disciples questioned him again about this,
and he said to them, 'Whoever divorces his wife and marries another is guilty of adultery against her. And if a woman divorces her husband and marries another she is guilty of adultery too.' "