No, I do not think that it is. If we are concerned with the health of traditional morals, we should be looking at ways to strengthen marriages of all sorts.
In order to argue that same-sex marriage is damaging to traditional marriage, we would have to say that it causes people to think less of the institution of marriage and to take it less seriously. I do not believe that same-sex marriage does this. Instead, I would argue that people who get married for a couple of months and make a reality show out of it, or those who marry multiple times and cheat on their spouses are the ones who reduce the level of respect that our society has for traditional marriage.
The real threat to traditional marriage is from our society in which people think that they should be able to do whatever they want without thinking of their commitments to others. It does not come from same-sex couples who do want to commit to one another.