A thesis statement is your initial presentation of the stand you are going to take concerning the topic of your paper, in this case same-sex marriage. Exactly what that statement says depends upon your opinion about the matter and how you are going to support that position.
What position are you going to present in your essay? Do you support or oppose the idea of same-sex marriage? Why do you feel that way? What support do you have for your argument? Have you collected articles, quotes, conclusions from surveys or polls, or other types of data or information that you plan to use in your paper to give strength to your stance?
You need to answer these questions before you can develop your thesis. Once you know what argument you want to present and you know how you are going to present support for that argument, then you can develop your thesis statement as a summary statement of your position. The first paragraph of your paper will contain your thesis and a brief introduction to each of the types of support you will use to try to convince your readers to agree with your opinion.