Some people argue that recognizing same-sex marriages will keep some heterosexuals from marrying. The theory, as best I can tell, is that the recognition will "diminish regard for marriage" (George Dent, The Defense of Traditional Marriage, 15 J. L. & Pol. 581, 623 (1999)), partly because treating traditional marriages "no better than a gay partnership" would "to most people . . . constitute not only the denial of a deserved accolade but a calculated insult" (id. at 617).
Now I'm pretty skeptical about this: I doubt that many people are put off from straight marriage by the fact that some percentage of people marries for bad reasons (e.g., marry someone rich and old whom you don't love and wait for them to die so you can inherit), and I don't see how the fact that some 2% or so of marriages would end up being same-sex would affect the remaining 98%.
Here, though, is a way we might try to test this (I realize that the analogy here is imperfect, but I think it can still be helpful): As I understand it, Catholics believe remarriage after divorce is wrong, and is in fact a form of bigamy. My sense is that while Catholics believe that religious marriage is a sacrament, they don't believe that the married parties are also religiously obligated to enter into a civil marriage. If they oppose civil marriage as an institution, they wouldn't be sinning by entering into the religious marriage but not the civil marriage.
Are there American Catholics who are so "insulted" by the fact that the civil law treats their marriage no better than a bigamous marriage of a divorcee, and who have so lost regard for the institution of civil marriage, that they refuse to engage in civil marriages? Or do they take the view that while some other people engage in bad or even immoral marriages, this wouldn't make their own marriage (both the religious marriage and the civil marriage) any worse? If the latter, then wouldn't we likewise assume that the behavior of opposite-sex couples won't be affected by the existence of same-sex marriages of which they disapprove?
Please do correct me if I'm misunderstanding Catholic doctrine here (which I well might be).