This is the hard part.
One of the reasons that it is hard is that we often think that recognizing the moral equality of, or even "tolerating" a class of people means assuming that they are the "same." This was the mistake of early feminism.
Whether homosexual relationships are the same as heterosexual ones is an empirical question, not a moral proposition. Homosexual relationships can be significantly different from heterosexual ones and yet have the same moral standing, just as men and women can differ in fundamental ways and have the same moral worth.
But just as the differences between men and women can, at times, justify differences in treatment of the one or special accommodations for the other, so might differences in homosexual and heterosexual relationships require differences in legal treatment.
Before considering how potential differences might affect the nature of marriage, we should acknowledge that it is not a sufficient response to these arguments to say that you know "some" gay couples for which the posited differences do not apply. These arguments turn on the general rule, not the exceptions to that rule.
Nor is it correct in assessing the objections to gay marriage to say that "two guys getting married won't affect my marriage." That's answering the wrong question. The question before is whether gay marriage will affect the institution of marriage. Not necessarily (not probably) your marriage, but marriages to come.
Having said that and repeating that I am still making up my mind, here is my best understanding of the argument that it will.
Maggie Gallagher says that if you change the public meaning of a social institution, you inevitably change the institution itself. If you extend marriage to relationships that are not fundamentally procreative, you will reduce the importance of procreation. Having children will no longer be the key purpose of marriage and the idea that mothers and fathers are the norm for children can no longer be advanced by the state, but, in fact, must be dismissed as discriminatory.
Before you dismiss this as scare-mongering, it is really no different than what a lot of advocates of same-sex marriage say. In the article that I linked to, Gallagher cites to a number of such people who argue either that marriage "has changed" or, if we permit same-sex unions, that it will. The only difference is that they think this is a good thing.
But maybe it isn't. If marriage is only about mutual affection and support; if it must be made to "fit" same-sex relationships, it seems naive to think that there will not be pressure to change it. Here are some possibilities.
Pressure upon the rule of two. This is not an argument that gay people prefer "open" or "triadic" relationships. But if marriage is not about a mother and father, then why should it be limited to two people (of whatever sexuality)? There are two ways in which this pressure might exert itself.
The first is a claim for consistency. Proponents of same sex marriage argue that they are not advocating for polygamy (although some are). But if marriage is intended solely to honor and support the relationships of mutual affection and support that people choose, what is to prevent extending marriage to polyamory or "ethically nonmonogamous" relationships?
The second way relates to the way in which same-sex couples have "their own" children (i.e., the natural child of one of them). They have to "bring in" a third party and they may well want - and the third party may demand - an ongoing relationship. This can happen for infertile heterosexual couples as well, but it remains an aberrational situation, not the norm that it might become for same-sex couples who wish to have children.
This may result in all sorts of pressure to change the rules of marriage to accommodate, if not more than two "spouses," then more than two parents.
It won't do to say, well, we already have multiple parenting situations as the result of divorces among heterosexual couples. For advocates of a marriage culture like Gallagher, that is a problem to be solved, not a principle to be extended.
Pressure on financial interdependence. Advocates of same-sex marriage argue that it is wrong to deny same-sex couples the "benefits" of marriage. This is a curious description of the financial consequences of marriage which involve as many burdens as benefits. One of the things the financial rules surrounding marriage do is burden the financially stronger partner for the benefit of the weaker. These rules are grounded in assumptions about what works best for couples raising children. If you apply them to relationships in which raising children is not the norm, then there is a potential "misfit" between those rules and what the people who are subject to them are likely to want.
Proponents of same-sex marriage don't deny this, but say that only those gay couples who "want" these burdens will choose to assume them by marrying. I'm not so sure.
I think it is undeniable that much of the push for gay marriage is about "sanctifying" these relationships. Much, although not quite all, of the claimed practical disabilities of not being married can be resolved by contract. Gay couples, understandably, want the legal imprimatur which says that their relationship is just as good as any other. But once these burdens begin to be felt - without the corresponding justification present in heterosexual marriage - there is likely to be pressure to change them.
Pressure on the notion that "mother" and "father" are best. I don't want to get into a battle of social scientists, but it is far from clear that we can abandon, for all time, the idea that, whenever possible, children ought to be raised by their natural fathers and mothers and that the parental rights of those natural mothers and fathers ought to be terminated only in the rare cases. While advances in technology and increased tolerance of alternative families have undercut this presumption, that is a rather recent development - almost entirely within my memory. I am not prepared to say that some of this may turn out to be an experiment that didn't work.
More fundamentally, I do not think we can say with any certainly that there is not something about mothers and fathers - a parent of each sex - that is better for children. We have very little experience with the alternative.
But if same sex partners - who can never be their children's natural mother and father - are allowed to marry, how will the state ever be able to encourage the natural parent - or even "two sex parent" - family? Won't private persons and churches who argue for the proposition that children need mothers and fathers be denounced as bigots and homophobes? Are we really ready to say that the way in which children have generally (I don't want to hear about remote tribal societies or decadent European nobility) been raised since about the dawn of man is no longer preferable? I sure haven't seen the evidence that proves that. I don't see how such evidence could even exist.
There is a bit more of this (and then we have to consider whether all of this is likely and what to do about the second sentence), but this is all I have time for now. I will continue the post that won't end later.