This is about the legal ramifications of marriage, not a religious ceremony. Marriage grants certain legal privileges to a couple. For example financial and immigration matters. Everyone is considered equal under the law and therefore everyone has the right to marriage.
Marriage is a highly sentimental institution that is hardly worth arguing over. Resolving this debate should be very easy, as it is simply frivolous and unnecessary to human life. If you care so much about marriage on either side of the debate, your priorities suck.
Marriage should not be legal for anyone. It should be abolished as a legal institution. It should be replaced by registered partnerships in secular states, both for same-sex and opposite-sex couples, since marriage is a religious institution.
A major hurdle is the institution and governance of marriage itself. Should marriage be governed by the state then religion should not be a consideration. Marriage is by nature a religious institution. The recognition of marriage within the state is not dictated by belief.
Same sex marriage renders no benefit to society as a whole (unbeneficial legislation), in many cases is undesired by the majority, encourages non-traditional, and potentially harmful relationships, and generally goes against nature (a relationship that does not have the ability to reproduce).
The lack of democratic input into the same-sex marriage debate (out of all countries that created SSM, only Ireland did so by referendum); Traditional wisdom, biology, social science