New Judge:
"the institution of marriage, which for centuries has been, for very good reason, the exclusive preserve of heterosexuals and which many people believe should remain so, now finds itself under threat of radical change which is simply unnecessary."
The reason marriage has been defined as heterosexual is because for centuries heterosexual relationships were the only kind which were really thought of as legitimate forms of love. Which, if accepted, does indeed make an exclusively heterosexual definition of marriage quite rational. Recent decades, however, have seen a huge societal shift regarding which relationships are seen as legitimate and which are not - most people (including yourself, if I've understood you correctly) now view homosexual relationships as perfectly legitimate and acceptable forms of romance.
Now that they're considered equal, homosexuals are asking for the right to participate in an important tradition of the culture they are born into. Just like everyone else (and you, it seems), they value marriage as an ancient celebration of love and wish to have the right to take part. And, actually, your average person seems supportive or at worst merely acquiescent/indifferent to the proposed changes. Other posters have shown quite extensively that there is no evidence of a clear majority who are actually opposed to gay marriage. What 'good reason' is there to bar them, exactly? Or to bar churches who are quite happy to marry them?