You know, maybe you're right. Maybe I just don't understand the argument, because everytime I address a different point I find out that that wasn't the argument either.
So, Zig. What exactly is your argument?
I'll try to make it as simple as I can. In fact, I'll even go a step further, and tell you how to frame a counter-argument.
1) The state has an interest in promoting and supporting procreation (note the indefinite article is used for "interest" - that's caused confusion before), and can advance this interest through marriage.
2) Marriage is not a right. It is a privilege, with benefits provided or enforced by the state. As such, it does not need to be extended to everyone, or to every possible couple.
3) There is a difference between same-sex couples and opposite-sex couples on the issue of procreation. Because there is a difference on this matter of state interest, treating the two differently is not unconstitutional.
4) There are costs associated with every benefit provided or enforced by the state, and those costs are borne by people other than the beneficiaries. The state should not place this burden on others unless a state interests is adequately served by doing so.
5) Same-sex couples do not serve the same procreation interest that separate-sex couples do, and so does not have a reason to extend these benefits to same-sex couples.
So that's the argument, in brief. Now, since you've had a hard time doing this yourself, let me show you the basics of what an effective counterargument would be (which attacks the second half of the last point above, which is the weak link in the argument):
1) Although same-sex couples do not serve the
same procreation interest as opposite-sex couples, they still serve
a procreation interest, through adoption, foster care, and artificial reproduction.
2) Procreation is
a state interest that is promoted by marriage, but (note the indefinite article used before) it is not the
only state interest promoted by marriage. Some other interest is also served, and served equally by same-sex marriages.
3) While state interests (the combination of procreation and any other interests) in promoting same-sex marriage may not be identical to the state interests in opposite-sex marriage, they are still sufficiently beneficial to society to warrant the associated costs. Same-sex marriage is therefore beneficial to society as a whole, and the privilege should be extended to cover same-sex couples.
I've only provided the outline, but there's a damned good argument to be made there, you've just got to actually make it.