So before we get started I need you to answer two questions- because they are essential
a) Are you against incestuous marriage?
b) If you are- why are you against incestuous marriage?
LOL You never "get started" in answering a question put to you. But as to answering your questions:
a) I am against destructive health practices, especially if it can harm others. Since "incestuous marriage" connotes procreation with harmful genetic effects on the offspring, I am against it.
b) See above.
Now will YOU "name one argument for gay marriage that wouldn't apply to marriage between close relatives?"
Back to the question then '
"name one argument for gay marriage that wouldn't apply to incestuous marriage"
So the same reasons why a brother shouldn't be marrying his infertile sister, is the same reason why a brother should not be marrying his little brother.
First of all a little description of marriage.
"Marriage is a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred,"
Justice William O. Douglas argued. "It is an association that promotes a way of life, not causes; a harmony in living, not political faiths; a bilateral loyalty, not commercial or social projects. Yet it is an association for as noble a purpose as any involved in our prior decisions."
Marriage is not just about procreation. If it was just about procreation then infertile couples would not be allowed to marry.
Nor are the prohibitions against incestuous marriage just about preventing birth defects. One state's law(I forget which) forbids marriages between a brother and sister regardless, but allows the marriage of first cousins- if they can prove that they are infertile.
So the law can recognize when an infertile couple are okay to marry- and when a couple- fertile or infertile- cannot marry.
So why the prohibition on a father marrying his infertile daughter or a brother marrying his infertile sister?
On the one hand there is the same argument against gay marriage- the 'ick factor'. But most of us recognize that the 'ick factor' is totally subjective and not a real rational to exclude someone from Constitutionally protected right of marriage.
The real reason that all incestuous marriages between a parent and a child, or two siblings is in place is because of the potential for imbalance of power making it so that the marriage is not one with full consent from one party. This is similar in ways to why there are laws against doctors dating their patients- again the imbalance of power.
A same gender couple of unrelated persons has no more of a risk of a power imbalance than a opposite gender couple of unrelated persons. An incestuous couple may truly want that 'coming together for better for worse' but society has deemed that the risk of abuse within that relationship is too high.