It may be a discriminatory stance, I am not certain since I am not an US citizen and haven't really dug into the details of what that law would imply.

It is not discrimination because there is not any individual being discriminated against. He did not use his position to discriminate against gay people for example.

He outright did the exact opposite and wrote that he was going to push for equality at Mozilla. I do not believe that he would discriminate anyone at Mozilla that was married to someone of the same sex.

I believe that a person can have professional courtesy.

This seems like a very 'pick and choose' definition of discrimination. I don't think it holds up — professional courtesy within one particular job and an outward political attempt, using his vast wealth and influence, to prevent a certain group of people from having certain rights, these don't have any relation to each other.