If the separation of church and state means anything, it must include the idea that people cannot be forced by the state to live according to the dictates of others’ religion. Just because one or many groups consider something sacred doesn’t mean that everyone must be forced by the state to do so. Just because one or many religious groups consider same-sex marriage a sacrilege doesn’t mean that civil marriage laws must be restricted to exclude that which some regard as sacrilegious.
It isn’t good enough for people to argue that same-sex marriage is against God’s will — it’s fine if churches teach this, but no government is under any obligation to legislate in a manner that is consistent with what what any church interprets God’s will to be. Marriage within a religion might be conceived as having been authored by God, but that is not and cannot be the starting basis for civil society. That would be the very essence of what it means to live in a theocracy.
In civil society, marriage is authored by secular laws voted upon by representatives of the people and as interpreted by the courts. We as citizens are the authors of civil marriage; religion no longer plays any essential role. Civil marriage does not exist in order to further any mandates from anyone’s gods. It does not exist simply to encourage and protect procreation. It does not exist because it is a “natural” function.
Civil marriage exists because society finds that it is valuable and worthy to encourage and protect committed, intimate relationships that are pursued over an extended period of time. As an institution, marriage helps provide legal protection and stability to human relationships that might not otherwise survive problems and pressures under more informal terms. Financial and social benefits are thus conferred upon marital relationships because their long-term stability furthers general social stability.
Ultimately, these secular issues are the only ones which should be considered when debating whether gay marriages should be recognized on an equal basis alongside heterosexual marriages. Would gay marriages help promote and support stable, long-term, positive relationships among gay couples? Yes. Would denying marriage equality to gays discriminate against them, putting both gay individuals and gay couples a serious social and political disadvantages? Yes. These are all the reasons we need to legalize gay marriage.