A fundamental problem with marriage is that it only comes in one size. As a legal relationship, matrimony is a monopoly product supplied by the government.
At the same time, however, as a personal relationship, the institution has unique, personal importance to those who partake of it. To some it even has deeply felt religious significance.
Thus, there is a mismatch between what is demanded of marriage and what is supplied. It is this imbalance that makes the prospect of same-sex unions a seemingly intractable problem.
Because there is only one legally sanctioned version of marriage, those who personally view homosexuality as a mortal sin (rightly or wrongly) are hostile to the prospect of sharing it with gay couples.
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2006/01/22/ING6FGOLVA1.DTL