Marriage is a right for anyone
Consenting adults should be allowed to marry their partner of choice. It should make no difference if they are straight or gay. While a license from the state is required, there is no such license requirement from a church.
A couple marries for a variety of reasons including economics, companionship, love, and sometimes – but not always - to have and raise children. Outside of actually having children, gay couples would be marrying for the same reasons. What’s not to think this is a good thing?
It turns out that marriage between men and women has not been the norm for all of human history, but a fairly recent legal structure. Even today, men are allowed to have multiple wives in several countries. And until recently (late 1960's), interracial marriage was banned in several states in the US.
Marriage, one might say has evolved over time and has a different meaning today than it did even 100 years ago.