I think that this is going to depend on the assignment's parameters. Are you required to lay out both sides? Do you need to choose one side of the issue? Are you able to find a middle ground? Defining what your essay has to have is going to be of critical importance before writing it. This will guide the writing process and also ensure that you are fulfilling the requirements of the task description.
I would offer one piece of advice given the topic. You have to ensure what role, if any, your own personal feelings will play in writing this paper. This particular topic, as can be seen by the two essays below, inflame passions. There is a particular intensity or vitriol to this topic that makes it difficult to argue it without being brandished some label by the opposing side. While this is a policy discussion, it also moves quickly into personal beliefs and even religious affiliation. In outlining how to proceed with this topic, make sure you are also clear on where, if anywhere, your personal emotions can enter into the topic. After surveying the literature out there, you should have a fairly clear indication as to what values each side holds in terms of if gay marriage should be seen as a civil right applicable to all or if marriage is an institution to which only heterosexual couples should have applicability. Upon this, and being able to clearly identify what you need to present in your essay, I think that the role of your own opinion has to be understood as you progress.