Okay, maybe I'll take another direction.
First let's see what marriage is, generally:
A legal contract (i.e. recognized by the state and/or a religious group) between two people in a romantic relationship with the purpose of one or more of the following:- legal, social, and economic stability;
- the formation of a family unit;
- procreation and the education and nurturing of children;
- legitimizing sexual relations;
- public declaration of love;
- or to obtain citizenship.
Source: Wikipedia.
Now, then, for what practical reason should we deny gay and lesbian couples contracts that seeks any or all of these things?