Because 'broadening' the definition in this case would be changing it. The definition specifically entails a union between a man and a women. That's the very criteria. If you were to include a man and a man, or a woman and a woman, then why not extend it to something else while were at it?
For the eleventh time......
The Supreme Court, as an agent of our government, broadened the definition of marriage to include unions between persons of differing skin colors only 39 years ago. My son would have been a felon had he married KK in 1966. She would have been a felon, too, at least in sixteen states.
Marriage hasn't always been just "between a man and a woman." In my lifetime, it's been more restricted than that, right here in the USA.