First, sexual preference is a choice, race is not. BUT whatever, I don't want to feed the trolls and argue that here.
Second, even if it is a so-called "protected" trait (a phrase which itself is an enemy of freedom), we are still talking about SPEECH. It is still not illegal to be a racist. It is likewise not illegal -- or deplorable, for that matter -- for reddit to decide they do not want to support racism. It is equally not illegal to celebrate redefinition of marriage, and completely within the rights of another to not support or advocate that redefinition.
These are not two different rights.