If you've had a vasectomy, and you're "fiancee" has had a hysterectomy, and you have no intention of adopting kids, I see no reason why the state should endorse your relationship.
I'm a libertarian, so I don't want any relationship of mine endorsed by the state (the example I gave is hypothetical). However, if you go around telling people such as the kind I mentioned in my example that they can't be legally considered married, you'll be deemed a kook, and rightly so....