I've actually written a paper in favor of same-sex marriage recently, so I'll throw out a few philosophical opinions you can explore in greater detail.
Alright so basically I wrote it for the Theory of Knowledge course I'm taking, and from what I can remember, arguing against or even mentioning the "sanctity of marriage" idea can actually be detrimental to your argument (this is an argumentative essay, right?). The religious debate regarding same-sex marriage is so subjective and opinion-driven that it's hard to win an argument with it. Whether or not it was "God's will" to allow homosexuals to marry is a really touchy subject, since then you're entering the religious debate. Alternatively, I think you should really focus on the ethical implications of allowing same-sex marriage. A profound, albeit controversial, comparison to same-sex marriage is segregated schools. Forcing homosexuals to unite under a civil union has certain parallels to the enforcement of blacks to attend specific schools. Not only are you creating a divide in the population, but these two institutions are not equal to either a marriage union or the schools that the white population attended. On an ethical level, this cannot be considered as morally acceptable, as one of the fundamental principles of ethics is to agree that all living individuals are equal. You can argue that this should not simply be racial or gender equality, but should also be extended to sexual orientation.
Being aware of certain counter arguments can also help you formulate opinions to support your thesis. One interesting opinion I came across was the notion that same-sex marriage should not be allowed due to the lack of biological connection between homosexual partners. According to this woman's opinion, this infringes on the biological rights of children, since there is an inherent biological connection between a man and a woman. You can acknowledge that this is true, but the premise of marriage is not to award people the right to procreate (which some people believe is the case), but rather to acknowledge the love and devotion between two individuals. As such, the feelings that homosexual couples feel cannot be disregarded as any less real or important than heterosexual couples, and thus marriage should be extended to same-sex couples. The religion debate is extremely touchy, but if you're a Christian/Muslim/whatever arguing this point, you can always mention that extending marriage to gay couples further strengthens the institution of marriage since it promotes the devotion between the two individuals.
Those are just a few of many ideas you can explore. Hopefully I won't attract anyone who is ready to refute the aforementioned opinions, as they're just that - opinions. Best of luck with your essay.