I think same-sex marriages should be legally recognized in the United States. I do not think the ability or intention to procreate a child should be a determinant or requirement, as this would also potentially open the door to legally preventing hundreds of thousands of heterosexual marriages from occurring each year as well. To paraphrase the man, I do not think that the role of a married couple should be to serve as brood mares to supply the state with soldiers, shoppers and worker bees. Furthermore, I do not think the common reasoning used in these discussions -- that the ability or responsibility to procreate healthy children should be totally equal among all marriageable classes -- should be the determining factor on this issue, in order to satisfy some requirement for consistency carried out to a nonsensical degree.
There is a large movement afoot to accord marriage rights to consenting non-closely related adults of the same gender, equal to that of heterosexual marriage, and I think that such a large movement exists should also be a determining factor. I don't believe that same-sex marriage rights should be denied because a majority of people simply wants to deny that right, especially given that their life, liberty and pursuit of happiness are not affected by this issue in the least. I especially believe that same-sex marriage rights should not be denied on the basis of religious reasons -- that is, that the sensitive sensibilities of certain religions would be violated by legally allowing same-sex marriages -- as passing legislation for such religious reasons is specifically prohibited by the First Amendment to the Constitution of the United States.
I do not think the discussion of gay marriage exists on the same plane as other non-recognized unions, such as between adults and children, between normal adults and mentally-impaired adults, between humans and animals, between humanoids and table legs, etc., because there is not the presence of at least two human adults that can consent to such union, and all parties to any marriage must be adults who have the ability to consent of their own free will.
I also think the discussion of gay marriage does not exist on the same plane as incestuous marriage, such as that between brother and sister, because there is a medical issue with bearing children. Namely, the state would be put in the position of encouraging procreation of inbred children, which would lead to well-documented mental and medical problems among a high percentage of legally-sanctioned inbred children, and this would be a socially irresponsible policy for the state to undertake. Furthermore, there is no significant (or even insignificant) movement to legally allow incestuous marriages, so from a societal standpoint, no large class of citizens is clearly being denied basic marriage rights. And to reiterate what I wrote earlier, I believe it should not a requirement that the argument regarding the procreation of children in incestuous marriages be exactly the same as that for homosexual marriage, for the purpose of legally recognizing such marriage.
As for polygamy and bigamy, as long as all the adults involved are not closely related and are also able to consent, I see no problem with legally allowing such unions. I do believe the mitigating factor here is to control for the prevention of masking situations such as white slavery rings under the guise of legal polygamy, but if an effective way can be devised to prevent such situations from occurring, I say, let 'er rip.
