Well, there are points and counter-points I can make for/against gay marriage.
But being a bit of a troll, I'll start with against XD
1- Against: What was the first reason behind marriage? (If anyone says "love", no offense; they are full of it) the prime reason was the making/raising of a child. IE the perpetuation of the human race. A gay couple, logically, can't partake in the first of the two parts of this process due to the lack of either a uterus or the necessary sperm. (I see you angrily getting ready to scream "sperm bank" and to this I answer a vehement "No" because of point #3)
2- For: There is always adoption, in all fairness, if you can't make your own, take the one someone else didn't want to/couldn't keep. Honestly a great point for, as there exist to many parentless children out there.
3- Against: Well, in what I am pretty sure is psych 101 (or psych some level), a child, be it boy or girl, requires a good role-model (bear with me on this one) for *both* sexes. That being the case, a gay couple, no matter how well-intentionned, can't ever give that child an appropriate role model for the other sex. Picture this; a couple of women get/raise a child, let's say a boy, although it's nice to say something along the lines of "sexual equality means there isn't a set role for both sexes blabla..." realistically, the boy will never truly internalize the norms of how appropriate sexual behaviour on a man's part is ideally portrayed. Meaning, although one of his... mothers (?) may be butch and so forth, he still will never truly, as both a young child and later on as an adult, get a firm grasp of what is truly expected of him as a man in things pertaining to both childbirth, appropriate conduct during courtship and the maintaining of the genetic ideal (more on that in point 5)
4- For: Simple point; It'll go miles towards making a more accepting and less bigotted society. Because when the child under such rearing conditions understands the true differences between the sexes, other than in actual childbirth, he may notice that in truth, no sex in inherently superior. This will probably make him more open-minded about other subjects such as race-relations and religion (don't get me started there though, I beg of you) to the point where he may become an accepting individual who judges people by individual merit as opposed to well... you get the point...
5- Against: (Disclaimer: I use the Hunting and Gathering dynamic of family in this argument and the next, also I attempt to offend no one as I do believe both sexes can do most of what the other can do. By the way, I don't actually *know* this stuff, it's mostly speculation that I engage in.)
Another simple one that I have somewhat tip-toed around up to now, this one may sound kind of over-zealous, but it's one that many people will think... A boy raised by two women will almost inevitably end up "effeminate". What I mean by this, however, is that the odds are that the boy will try and emulate the more "fatherly" of both patents which, in a straight couple, will lead him to try and play sports, the value of physical labor and eventually, as he gets older, he will want to learn the skills his mother can teach him due to simple practicality. Of course this only applies to a boy raised by two women etc...
6- For: Counter-point to #5; in this day and age, no job is mutually exclusive to either sex, a boy raised by two women (maintaining my previous example) who would be encouraged to be athletic because, let's face it, any parent worth their salt will at least encourage physical health, a hard days work and so forth whereas a girl raised by 2 men will be taught by one of the men how to cook (because if neither knows how to cook, we are looking at a poor example of a couple...) while probably still internalizing the previously mentionned "man's values" thus leading to a well-rounded individual without necessity of the second sexed role-model.
7- Against: This one isn't mine so much as one I've heard before, so please don't get as mad as I was when I heard it; A boy raised by two men will imitated his dads and become gay as well, which would only encourage more boys to become homosexuals in the long run.
Same for a girl raised by to mothers; she'll see men as disposable and become a lesbian.
A rebutal to this could be that(FYI; I cannot truly know and am speaking more out of mouth-to-ear knowledge, some of which may be wrong) being gay is not a matter of choice, in fact, it would seem it's somehow, I presume, genetic as opposed to developped by nurturing conditions. However, the argument here is reliant on the Nurture aspect of the Nature vs Nurture debate, an argument for another day...
8- For: I'll end on this one, which is aimed more to the over-zealous straight crowd; if you make gay marriage allowed, this would not only mean that society accepts homosexuality as less of a stigma, allowing those in denial to come out, it would also allow the closetted ones a way to be lumped together. Thus, if in fact, they are what what the biggots would call a "lesser branch of humanity" the basic lesson of Darwinism would imply that, since they can't procreate, they wouldn't be able to "perpetuate their kind" and eventually would die out. Whereas keeping the whole biggoted view would only encourage them to hide in a "normal marriage" and allow them to make children who (if being gay is truly a genetic predisposition) will in turn either carry a "latent gay gene" or outwardly turn out to be gay.
In conclusion, although you haven't noticed, I'm for gay marriage. It's a great idea. Plus, if there is a hateful close-minded biggot out there who would disagree with me, read point #8 and shut up, even *your kind* will be appeased by the logic.
Final point, is anyone wants to imply that I may be preaching against, I mark that not only do I support quite a few friends of mine who are gay and coming out/already out, I was merely playing "the devil's advocate" with all the against points because a wise man once to me; True Wisdom Is Being Able To See Both Sides Of An Argument.