Prometheus
Semper Perconctor
However, I would say that I have produced a legitimate secular argument as the OP requested although as I have been forced to admit, it isn't one I agree with.
I will concede you have proposed a good argument. However, all it has convinced me of is that there is a flaw in the government's perception of which relationships should receive benefits and which should not. It has not convinced me of any immorality in homosexuality. I realize this was not your intention to begin with, but I still wish to point it out.
Furthermore, even if I were to concede that marriage itself is not a human right (I'm still debating this with myself depending how you define marriage), many of the legal rights and benefits currently granted by marriage are human rights which, as of now, are either unnecessarily difficult or impossible to contract for otherwise.