I agree. The problem I have with the gay marriage opponents is that their arguments seem to boil down to either a) It was always done this way so we can't change it or b) My religion says this is a marriage so we can't go against that.
In the case of A, we change things all the time. At one point, "people were always kept as slaves" but we realized that was wrong and changed that. At one point "women and non-whites were always kept from voting", but again that was changed. In addition, things weren't "always" the way people think they were always. Taking multiple wives and having concubines was pretty standard practice (if you could afford them since women were basically regarded as property) for much of history. Just try arguing "but it always was this way" when you try to apply to marry a second wife without divorcing the first one.
In the case of B, a person can't use his or her religion to impose restrictions on another person. I don't say that nobody else can eat bacon because I'm Jewish and bacon isn't kosher. Go ahead and eat all the bacon you want - or don't eat any. It's none of my business. Similarly, a person can't say "my religion forbids gay marriage and therefore even people who don't observe my religion must obey this rule."
As a side note: nobody is saying that priests and rabbis HAVE to marry any two people who walk up to them. If a rabbi doesn't want to officiate in the marriage of a Jewish man and a non-Jewish woman, he doesn't have to. He's free to decline and someone else will officiate. The same will go for priests who don't want to officiate in marriages between two men or two women. However, this ruling does mean that somebody acting as an agent of the government can't say "I don't believe in gay marriage and thus won't recognize it as real." A Justice of the Peace can't refuse to marry two men. An IRS official can't deny that a woman and her wife are filing jointly. They need to follow the government's rules, not their own personal religious rules. Then again, this is true for a lot of their job so anyone who can't deal with this should probably find some other line of work.