You're conflating an act, getting married, with a state of being, being gay. You could use the same justification of marriage being an "opt in" event to justify refusing to serve an interracial couple or literally any protected class.
Similarly, eating at a restaurant is an "opt in" event. Should you be able to discriminate at will in that circumstance?
I am by no means an advocate of Mill's liberalism, but according to that philosophy espoused everywhere else but the case of a business refusing access to black customers seems to me inconsistent.
Similarly, eating at a restaurant is an "opt in" event. Should you be able to discriminate at will in that circumstance?