One of the most common arguments against same sex-marriage is it will threaten the institution of marriage. Please logically explain how two people making a commitment to a long-term, loving relationship threatens marriage. 

Some say it’s about procreation: Then why are infertile couples, couples who never want to have children, or elderly couples allowed to marry?

Or is it the Bible? I thought that freedom of religion implied the right to freedom from religion as well. The Bible should have no standing in American law.

Others say it’s the start of a slippery slope. So why haven’t the countries and states that have had same-sex marriages for many years been confronted with this?
 
We have a constitutional democracy, which is designed to protect minorities from the tyranny of the majorities. Gays and lesbians are a part of the fabric of our American society. Can anyone provide any logical reason to deny these fellow Americans the benefits of entering into a long-term committed loving relationship?