@viviane:
You need to use another dictionary. Dictionary.com is more of a Wikipedia, not a scholarly dictionary. Oxford, Merriam-Webster, etc. are examples of scholarly dictionaries.
Quoting one union of a same sex couple is cherry picking a rare instance.
The fact is that the term marriage is defined as between man and woman. So, why are same sex couples trying to change the definition.
Actually, answer this question!
Why would same sex partners WANT to use a term that so closely associates their union with thousands of years of same sex union?