I think you can come up with those on your own, given your vociferous disapproval. Let me give you what I believe are fundamental reasons why gay people SHOULD be able to get married.
Fact one. Gay people are humans, just like straight people.
Fact two. Article 16 of the United Nations Declaration of Human Rights states clearly that "Marriage is a human right".
Fact three. Based on the above facts, logic dictates that as humans, gay people should be able to get married.
I'm actually helping you here.
If you can disprove all three facts above, or put another way, if you can prove that:
1. gay people are NOT humans
2. there is no such provision in the UN Declaration of Human Rights
3. since gay people are NOT humans, they are excluded from the list of human rights, and therefore logic would dictate that they cannot "receive" the rights "awarded" to straight people, the only ones considered "humans".
then you will have your essay complete and correct!
Until such time, don't bother crossing our paths with your hatred. I assure you, not all of us will react so benignly.