How does same-sex marriage threaten traditional marriages. Heterocouples will still get married (and get divorced very quickly).
The sancitity of marriage is already trampled on. Most marriages end in divorce within several years anyways. Very few people try to uphold their vows and work out their problems. At least gays would have a chance to prove that they are normal people. Hell, they probably just want the tax breaks anyways, not some reverand reading their vows. The sancitity of marriage is gone and has been gone for a very long time.