You're confusing roles within the relationship with roles in society here. If a partner is dominant/ submissive/ butch/ femme/ girly or whatever else that doesn't describe what the social function of the relationship is.
The opinions of bystanders, do not affect this function either. A marriage between a black and a white person is perceived differently in many countries than one between two people of the same skintone. This does not change the function.
One can define the social function in many ways: to raise children, to afford people with stability in their life, to lower the cost of living (as married people usually share accommodation), companionship, tax benefits, certain rights of inheritance.
All of these functions can as easily be attributed to gay relationships as they can to heterosexual ones.
You're argument is easily reversible too. Say we did create an institution that was equal to marriage in all but name. It looks like a marriage, smells like a marriage, for all purposes is a marriage: how exactly is calling it something different not discrimination at that point?
Now, I personally agree with you on not wanting to have anything to do with traditional gender roles and the values that some people attach to the word marriage, but that feels like a completely different debate to me.