Not a cop out at all. I once had a very nice chair. During a long distance move, a leg was broke off by the moving company. I kept the chair for almost two years while trying to find a guy that could fix it. Finally I threw it away. By design it was a chair, when the leg broke, it was still a chair. It did not become something else.
Heterosexual unions are reproductive by design, even if one is broken, they are still reproductive by design. By design same sex unions and pedophile union are sterile. Should these sterile relationships be included in your definition of marriage? Can I assume by the bolded part of your post that the answer would be yes?