What this comes down to is 'them' and 'us'.
Society as a whole has made massive strides in the acceptancec and understanding of homosexuality in the last fifty years, but the notion of homosexuals as being outsiders in society still persists in cultural attitudes.
This is what manifests itself in the argument about marriage - the church does not want 'them' to enjoy the same status is 'us' because it still believes that to be homosexual is to varying degrees 'different' 'substandard' 'subversive', 'unatural' etc. etc.
So to sanction the concept of marriage for gay people is to be seen as agreeing to their lifestyle and practices, which powerful instituions, including the church, are not keen to relinquish.
But the march of equality and acceptence is relentless, that particular genie is out of the bottle, and like all genies, it cannot be put back. Marriage for gay couples will come to be - maybe not yet, but it will happen.