Originally posted by lucifershammer
Not as trivial as you suggest. By shifting the meaning of marriage away from an institution with a dual purpose (union of two people, rearing of children) to an arrangement with a single purpose (union - or maybe just legal privileges), you change the terms of reference for all marriages - same-sex and different-sex (I prefer those terms to 'ga ...[text shortened]... arily the biggest threat to "traditional" marriage - but it is a threat.
You were talking about ideas, not institutions. So, your claim was trivial. Now you are talking about something different: the effects on the institution of marriage if homosexuals are allowed to marry. While I'm not quite clear on just what an institution is (e.g., a set of practices, a set of norms, a set of attitudes, some combination), the only way your argument gets of the ground is if you can provide evidence that some actual harm to actual persons will result from allowing homsexuals to marry.
Your argument seems to be as follows:
1) The purpose of marriage is unification and procreation.
2) If homosexuals are allowed to marry, then the purpose of marriage will change to merely unification.
3) If the purpose of marriage is merely unification, then heterosexual couples will be more easily able to divorce.
4) If couples can more easily divorce, then children will be harmed.
Now, I have no idea why we should accept premise (1). While you may think that marriage has some divine telos, your justification for this claim is only as strong as your justification for the metaphysical implications of this claim. Of course, there is simply no reason for secular folk to grant that some gaseous vertebrate in the sky imbued marriage with some dual purpose. But, you could claim that the purpose of marriage is set by the state, or by the beliefs of the majority. But why should we believe this? The reason a state sanctions certain relationships may be to encourage procreation, and some people may get married because they want to have children, but this is perfectly consistent with the marriages of other people having different purposes. Your marriage may have a dual purpose while mine only has one. The fact that my marriage has one purpose doesn't magically remove one of the purposes of your marriage, nor does the fact that your marriage has two purposes magically imbue mine with an extra purpose.
Further, for precisely this reason, there is no reason to accept premise (2). If homosexuals are allowed to marry, it is perfectly consistent with their marriages having one purpose that heterosexual marriages have an additional purpose. Further, it is simply wrong to claim that homosexual marriages can only have one purpose. Homosexual couples may have children, either through adoption or through technologically aided means.
Further, there is no reason to accept premise (3). It is perfectly consistent with allowing homosexuals to marry that all marriages are also rendered more difficult to dissolve. There are any number of legislative means whereby this could be accomplished. Also, pace the comments above, there is no reason to think that if homosexuals are allowed to marry (and if their marriages are somehow by their very nature of a single purpose) that this will "bleed over" into heterosexual marriages. You certainly have provided no evidence for this claim, and the evidence does not in any way suggest that in those locations where heterosexuals can marry there has been an increase in heterosexual divorce attributable to homosexual marriage.
Your "devaluing" claim is bull. As it stands I could legally get married to my former roommate. So what? The same reasons that people don't in general go around marrying roommates and friends apply just as well to homosexual marriage. By your logic, we should prohibit heterosexual marriage because Tom and Jane could get married for a tax break.
Homosexual marriage poses absolutely no threat to heterosexual marriage. This is simply magical thinking on the part of those with an inadequate appreciation of the nature and power of the bond that homosexual couples can form with each other.