One issue that I see as central to the question of harm or infringement of rights by same sex marriage is the issue of gender, specifically gender essentialism. I would define gender essentialism as: the belief that there are uniquely feminine and uniquely masculine essences, specifically uniquely masculine and feminine social and behavioral traits, not referring to basic physical differences, which exist independently of cultural/social conditioning.
In my opinion, it is possible to reject gender essentialism and still believe that there are differences between men and women. Gender essentialism is more about behavior and psychological hard-wiring than it is about basic physical, biological, or neurological differences. For my purposes here I will assume that men and women have basic physical and psychological differences, but that those differences are averages, not absolutes (e.g. men are usually stronger and taller than women, but some women are stronger and taller than some men).
I would represent both social and physical traits, reproduction excepted, on a linear scale- meaning that for each trait there is a line, say aggression/compliance for example. On this line, the average female position and the average male position may fall weighted towards one end or the other, but individual positions for either gender could be anywhere on the line. I do not believe that men and women are opposites. I do believe that there are clear and significant statistical differences in physical areas such as upper body strength, though again they are not absolute- uncommon though it may be, some women are stronger than some men. (Also, training and conditioning have a huge effect on situational outcomes- for example, while assuming the same training and fitness level, men seriously have a significant advantage over women in the physical strength department, a woman with superior skill and training can absolutely be more than a match for a man who does not have that training.) In the case of social traits/behaviors, however, I believe that there is as much difference within genders as there is between them; that is to say, there is as much difference, or more perhaps, between two random women as there is between your average man and average woman. I do not think that all men are/should be dominant in certain traits, or that all women are/should be dominant in others. I see anecdotal evidence in my own life which supports my belief that I have more in common with males who share my personality type than with females who do not.
Also, if gender based social behavior was a biological, undeniable constant of the human experience then, except for the tendency towards male rule and oppression of the weaker by the stronger which I believe was the result of the fall, I would expect to see this gender based behavior as a constant across cultures and times, and socio-economic status. I do not see this- in fact, quite the opposite. The cultural norms for acceptable gender behavior may well be consistent in modern, western society, but that is not at all the same thing. For example- consider the view of women's sexual nature at the time of the reformation contrasted with the Victorian era. I would posit that gender norms in history as a whole are actually quite varied and fluid, but that is another post entirely. Also worthy of another post is the influence on our cultural perceptions of gender- not of fundamentally christian teaching, but of greek thought and philosophy.
The reason this issue is at the center of the debate over same sex marriage is, first, that it is reasonable to suppose that both a stable heterosexual couple and a stable homosexual couple could hypothetically bring the same backgrounds, education, experience, moral code, religious knowledge, et c. to their marriage and their parenting- really, the only difference is the gender of one of the parties. Are the genders so unique that a family or couple will lose a vital part of its essence if one gender is missing? No, I really don't think so. May they be different? Yes; but will they "miss out" on something to such a degree that we must refuse to legislate in favor of their marriage in order to protect society and any children they may have from this horrible loss? I really don't think so. This is, of course, merely my opinion; I am aware of no comprehensive studies of children/families/marriages which specifically compare same and cross gender couples, with other major variables being equal, to determine which families, spouses, and children are healthier.
Secondly, while gender differences/gender essentialism and gender roles are not the same thing, they are related- rejecting gender essentialism leads to the questioning of rigid gender roles and societal systems that require them in order to continue functioning. Unless gender-based prescriptive behavior is purely theological or ritual, with no basis in practical good or expediency/efficiency, believing that there are actually not rigid, biological, hard-wired social/behavioral ideals makes implementing rigid gender roles which are based purely on gender without regard to competency seems rather silly. In other words, why make mommies staying home/daddies working a moral/civil prescription if daddies can be just as nurturing as mommies and mommies really have no trouble navigating the wide, scary world of outside careers? Also, if men and women share social traits and differ more from opposite personalities than from opposite genders then the male rule, headship and female submission doctrines become at best a theological ritual with no basis in practicality. I honestly think that some of the more vitriolic rhetoric I've heard condemning same sex marriage comes from a place of fear- a fear that the systems that have supposedly kept society intact will slip away, or that a privileged position will be lost, or that the tidy boxes that we as christians are supposed to fit ourselves into in order to be "Godly" will go away. I honestly think that that would be a good thing, when it comes to gender roles. We'd be left with freedom, relationships, and personal responsibility, and we wouldn't have to throw out godliness, christlikeness, or holiness to do it.
But.....roles and tidy boxes are easier. Really. They may not fit everyone, and they may chafe unbearably for some, but you're good to go if you can fit into them! It's certainly easier to have litmus tests and checklists for godly masculinity and femininity than it is to have to figure out how to hear the Holy Spirit, follow the guidelines Christ gives all Christians, and find and walk in the purposes, giftings and callings that are a part of each of our unique makeup as people and as followers of Christ. I am not saying, of course, to throw out all rules or commandments- but I think it would behoove us to resist making doctrines and prescriptive procedures of things that are not compatible with the teachings of Christ, the overall message of the gospel, and that don't work.
So- why is it, really, such an issue for gender to take less of a front seat in marriage and family issues? I have yet to see evidence that even reasonably assumes, let alone conclusively proves, that gender is the thing that makes such a difference to family and society that we must legislate it, even for those who believe differently than we do, as a moral and civil harm.