You are aware that it is the lawmakers that make the laws concerning marriage right?
Thus the legal definition of marriage is whatever the law says. If your constitution gives equal protection to all citizens, then you must change the laws to fir the constitution.
Then the definition of marriage will include same sex, just like it now includes different skin tones.
Then gay marriage will be a part of marriage(TM)
You sound as if the definition of a marriage is immutable.