Well reasoned and well stated. [Doffs cap to telerion]None of those arguments is at all persuasive. My understanding of the legal point here is that the government must have a compelling reason for treating homosexual differently than heterosexuals. There has to be a substantive state cause, otherwise it's discrimination.
Reason 1: If the ability to create new taxpayers were the substantive cause, then the state should not recognize heterosexual marriages between infertile couples, including old people. The state however does recognize these marriages. If this condition is only applied to homosexual couples then it's discrimination.
Reason 2: In any marriage there is thing or collection of things that that couple unique. This reason doesn't put forward any case for why homosexuality should disqualify a couple from being married. It just begs the question. Replace "homosexual" with "interracial" and the lack of substance should really jump out at you. (Plus it's the sort of statement that people in the 50's and 60's might actually have made against interracial marriage).
Reason 3: This also isn't an argument against gay marriage specifically. It's an argument against all marriage. Since the state still recognizes heterosexual marriage, and there's no reason offered herein that explains why gay marriage should not also be recognized.
Reason 4: This is argument is not only in compelling but it's also chauvinistic because it assumes that women don't own businesses. Does the state have a problem with mergers between men and women? Do we seen this abuse of marriage occurring now? At best this is an argument for reexamining business law as it relates to marriage.
So while these four reasons are certainly secular, none of them addresses the central question, "What compelling, substantive reason does the state have for recognizing heterosexual marriages and not recognizing homosexual ones?"