The fact is, none of us would be here without heterosexual behavior. Even if we perfect cloning or any method of reproduction that doesn't require something from both sexes, we wouldn't have arrived at this point in history if it weren't for heterosexuality.
People who have engaged in homosexuality have contributed to society - as have faithful Christians, or any number of people who have engaged in behaviors with which any of one of us might disagree. But homosexual ACTS, unlike heterosexual acts, do not contribute positively to society. Some of them are particularly effective at injuring participants and spreading disease.
As a society, we do not have the same interest in same-sex unions as we do both-sexes unions.
http://walrus.blogtownhall.com/2008/05/17/why_marriage_matt ers.thtml