Asking this question is kind of like asking, "What should I say if I want to participate in a conversation about same sex marriage?" All the books and articles we read are just like a big conversation, so when you participate by writing this article, what will you say?
What point do YOU want to make? Google this:
arguments for and against same sex marriage
I think you will do well if you write a theses like this:
Some people argue that the institution of marriage is intended to bond one man and one woman and that the definition of marriage makes same sex marriage improper, but this paper is intended to show that the modern definition of marriage should be one that allows for all lovers to experience it.
I also think you might have to write about the 2 different definitions of marriage -- the church definition and the state definition.
Remember to start every para with a topic sentence.