Indeed. If they were, we wouldn't be talking about gay marriage now.
People suggest that allowing people of the same sex to marry would destroy the institution. I say, gay marriage would only be the final stage of destruction of the institution that began with the concept of no-fault divorce.
To wit: Once marriage was easier to legally escape than a health-club membership, the differences between marriage and shacking up began to evaporate. Now that people with normal sexual inclinations can shack up and get almost all the benefits of marriage, gays can say, "Hey, we shack up too!"