Good question Fire.
There’s several reasons why it’s not possible.
Marriage is a Divine institution and its deffinition can be found in genesis between adam and eve.
Same sex couple does not fit the definition.
They are unable to procreate (“be fruitful and multiply” ), man and woman are compliementary and are able to reveal much of who the spouse is (after looking at eve adam says “this one at last is bone of my bone and flesh of my flesh” )
In same sex couples, you don’t get any of this.
Also, same sex “marriage” is against natural law. Our bodies were created in such a way that we can tell how they are used and for what purpose (biology tells us this) …the sexual act was obviously made for a man and woman (by the way the body is designed) .
Also it was God’s intention to make us in His “own image and likeness” …God as we know if trinity (a family) . A man and woman are able to actually create a family and work with God in His procreative process and produce family = image and likeness of God in the world.
This is just a basic idea. I hope it helps…if you have any more questions …don’t hesitate.
I HIGHLY suggest looking into “theology of the body” google Christopher West…amazing stuff!