Women can only marry men, and men can only marry women. There is no gender equality there. Judge Walker noted this specifically in his decision, making the observation that in the case of marriage, discrimination by sexual orientation is equivalent to discrimination by sex.

I'll also note that your inevitable response to this, that marriage is not about two consenting individuals choosing to marry each other but in fact relies on specific gender roles, is both antiquated and unreasonably limiting to personal liberty. This is a purely conventional argument, so let's note up front that appeal to tradition is a logical fallacy.