In general, I don't understand why they need to have a marriage anyways. Why not just get domestic partnerships granted at the federal level? They'd get much less flak.
The ideal solution is domestic partnerships at the federal level -- for ALL people, regardless of their combination of sex. However, the federal government is never going to relinquish the power of marriage. We're stuck with the state regulating a religious issue. Within this framework, the difference between domestic partnerships is mostly emotional. It is incredibly insulting to suggest that something "separate but equal" is a good solution. Beyond that emotional issue, it is unlikely that something intended to be different in name only would only be different in name only.
That ignores the fact the traditional societal supports for marriage are all based around the fact that it is a generative relationship. That is why more resources are devoted to it.
Sure, maybe same sex relationships can be artificially generative, but this is no where near the capability or likelihood to equal the basis for traditional marriage.
So, same sex relationships should not have the same benefits that are accrued by a generative relationship, since they do not require them and are thus just squandering resources.
Emotional issues are important, but they should not be the bottom line in a situation that clearly has a functional basis. However, the pro side seems to be mainly emotive in their driving arguments.
UPDATE
@downvoter, if equality isn't a fuzzy premise, where is the line drawn between things that should and should not be equal?
For instance, why shouldn't everyone have an equal opportunity to participate in the Olympics? Why shouldn't everyone have an equal opportunity to be a CEO on Wall Street?
It's because they do not have the functionality that these positions require, as mynameishere pointed out regarding marriage. Claiming equality merely glosses over these important distinctions and does not bring any substance to the argument.