The argument that same-sex marriages threaten, attack, injure, or invalidate heterosexual marriage makes no sense at all....because it lacks any mechanism. If you ask how sugary soft drinks threaten/attack/injure tooth enamel, for instance, dentists can give a mechanism and explain how that works. If you ask how alcohol injures drivers' ability to drive well, doctors can give a mechanism and explain exactly what alcohol does to the brain to make it work less well. Safety engineers can explain the mechanism by which this or that industrial accident happens....psychologists can explain the mechanism by which group solidarity becomes bigotry.
But ask opponents of same-sex marriage how, precisely, same sex marriage injures heterosexual marriage and they can't give an answer. No mechanism exists to move from "Bob and John married," to "Bill and Susie's marriage was destroyed." Marriages succeed or fail because of the characteristics, choices, and behaviors of the marriage partners....not the sexual orientation of the neighbors or co-workers. If someone chooses to use another person's sexuality as an excuse to be a bad marriage partner (and this applies to any someone--straight or gay) it's just an excuse, not a reason. The only people who can ruin a marriage are the people in it. The only people who can sustain a marriage are the people in it.
Will same-sex marriages prosper? I hope so. Initially, I'd expect that longstanding gay partnerships, now formalized as marriages, would beat the odds and outperform heterosexual marriages, on average. But I would expect over time that marriages of gay persons would show a similar pattern of success/failure to heterosexual marriages, because the causes of both success and failure are common to the upbringing of both. A healthy marriage requires both partners to have courage, integrity, and generosity...cowardice, dishonesty, and selfishness in either partner can deepsix a marriage.
So hurray for New York and I hope the movement spreads.