I apologize in advance for how dry this treatment is but came after a debate with someone who used what seemed to be a very logical argument and so I had to dig into the logic as much as possible to treat it fairly and also refute it.
The argument of many Christians for why Homosexuality means the destruction of marriage seems to run thus; If A is to B as A is to A then A must necessarily equal B, which is false, therefore A cannot be to A as A is to B.
Or in other words, if the marriage of a man and a woman can be equated with the marriage of a man and a man then men must be the same as women, which is not true. Therefore there can be no such thing as gay marriage and no matter how one might like to call gay relationships marriage it would be, like Caligula calling his horse a general.
The presupposition is that A and B are species designations and contain no variance within them. If we consider them as a genus then his conclusion does not follow. For if A is considered to be even and B odd then we are perfectly able to say that 1 is to 2 as 2 is to 4 without ever having to conclude that even is equal to odd.
The supposition has not been proven and could be thought to have been chosen in deference to the argument rather than its complete necessary validity.
The definition of men and women need never be tied to their relationship between each other but only in relation to themselves. We might say that men share certain similarities and women share certain similarities, which does not necessitate that their relationship with one another will bare on their similarities, that is, their definition.
My basis for this idea is that the definitions of men and women are experiential phenomena and therefore contingent. No necessary and universal truth may be derived from experience; only specific judgments that are always open to doubt.
To argue anything on the basis of the definition of men and women is to argue from an uncertain foundation, in my understanding. It may be that those who argue that Homosexual marriage cannot be has these definitions perfect and from a perfect source and that theirs is absolutely correct but their argument does not allow me to judge that.