I've explained the societal benefits of marriage repeatedly: stability and order. Neither is contingent on heterosexuality.
You have yet to explain the harm of including homosexual couples in society's definition of marriage.
Stability and order could be considered a benefit of traditional marriage. However, given that liberals believe a person should be able to divorce for any reason at any time "stability and order" not found. Take a look at
Marriage is just a property contract as proven in any divorce court. Though a wife is no longer considered the property of the husband anymore. Gay marriage is about getting the same properety rights as same sex.
Pretty sure unmarried people are allowed to own property together.
What gay people want is things like tax-free inheritance(cost to society), access to a partners SS benefits (cost to society), etc.
EDIT: So the question is if "marriage" is just a way to give you a way to get benefits from the government for the person you are screwing (or not screwing as that is not a requirement) why should such an institution exist?
Besides with gender being a surgical (with a little advance in cloning a fertile change) choice, what is the big deal?
Marriage is based on biological sex. Not the whatever I feel like I want to call myself "gender" idea.
Next cause? How about the equal rights for Women Amendment?
Why would liberals fight for women to have fewer rights? :hmm: