Why are you equating marriage with sex? There is a huge difference between the two.
And why are you applying Old Testament law to people to whom it wasn't written? Leviticus was an extensive set of laws that was written for the Jews, not anyone else. If you apply the law then you must apply all of it, correctly, to the people to whom it was written. Jesus is the -> end <- of the law for those who accept Christ. We are no longer under the old covenant. The Levitical law as never written for the non-Jews, i.e., the Gentiles.
You are indicating that the old covenant is the Levitical Law and I assume that you are under the new covenant spoken of by Jesus;
Mark 12:29-31King James Version (KJV)
29 And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord:
30 And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment.
31 And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.
Jesus said:
John 14:15 If ye love me,
keep my commandments.
The first four commandments is how we love Jesus. The last six are how we love our neighbor. So the only way this is a new covenant is that the Jewish nation at the time of Christ had perverted the first four commandments by inserting many "laws" and traditions that are not in God's Teaching and adopting Greek Mythology.
Matthew 21:12
And Jesus went into the temple of God, and cast out all them that sold and bought in the temple, and overthrew the tables of the
moneychangers, and the seats of them that sold doves,
They did not love their neighbors as we are told;
Acts 10:28
And he said unto them, Ye know how that it is an
unlawful thing for a man that is a Jew to keep company, or come unto one of another nation; but God hath shewed me that I should not call any man common or unclean.
The way this is a new covenant is where if is written according to Jesus:
Hebrews 8:9-10 King James Version (KJV)
9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord.
10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
So God's laws are still in effect for his people. Therefore the opposite must be true. Those that have not his laws are not his people.