At a trivial level I think people are misusing Kant's categorical imperative - if everyone got same sex marriages the human race would go extinct, therefore same sex marriages must be bad. Silly argument, which is why few people make it explicitly.

I hear a fair bit in my O community about same sex marriage being part of the 'homosexual agenda'. Of course, that agenda allegedly includes promiscuity of exactly the sort that marriage is meant to limit. Also, as someone pointed out, in politicly conservative Jewish circles 'gay activists' seem to be as powerful as 'the Jews' are in anti-Semitic circles.