1. The main purpose of marriage is companionship
Then God said, “it is not good for the Earth Creature to be alone.  I will make a fitting companion for it.”    Genesis 2:18

2. A lifelong companion has to be someone compatible
A lifelong companion has to be someone compatible. Gender is not important.
 The Earth Creature gave names to all the cattle, all the birds of the air, and all the wild animals. But none of them proved to be a fitting companion.  Genesis 2:20

3. We are all made equal in the image of God
So God created mankind in his own image. In the image of God he created them; male and female he created them (Genesis 1:27)

4. If marriage is a ‘remedy for sin’ for opposite-sex couples then it is equally so for same-sex couples
But if you cannot control yourselves, then you should marry, for it is better to be married than to burn with passion.   1 Corinthians 7: 9

5. The diversity of human nature and sexuality is a gift of God
I would hope that everyone could be like me. But we all have our own particular gifts from God. One has the gift for one thing and another has the gift for another thing. 1 Corinthians 7:6

6. Human are designed for procreation
God said, “go forth and multiply. Fill the earth and subdue it.”(Genesis 9:7)

7. The essence of marriage commitment of heterosexual union
   Now, she will be woman, and I will be man, because we are of one flesh.  Genesis 2:23

8. The first example of a couple was a heterosexual couple – Adam and Eve
“she will be woman, and I will be man, because we are of one flesh” (Genesis 2:23)

9. There are no examples of homosexual relationships in the Bible.

10. God destroyed the cities of Sodom and Gomorrah because of their sexually immoral ways (and other sins)

11. It is the Natural order of things, as revealed by God.
St. Thomas Aquinas presented the ‘Natural Law’ Argument, which states rules that were revelled to us by God. One of these rules is to ‘continue the species through reproduction’.