"...."marriage," by definition, meant one man, one woman ..."

Actually no. The definition restricting it to one man one woman unions was only introduced into USA law 2004/5/6 across numerous states in a frantic attempt to avoid courts making similar findings to those of the Massachusetts Supreme court ruling.
Prior to that it had always been expressly defined as between "two people", which is what triggered the Massachusets challenge.

The fact that only opposite sex marriages were performed in the past, does not mean marriage was defined as only between opposite sex couples, it simply illustrates that couples who were not opposite sex were being denied a fundamental right.

The evidence of the existence of discrimination is not proof that the discrimination was justified or justifiable.