One side of the marriage equality 'debate' might consider it to be a political or religious issue - which is a large part of the problem, really. Most people on the other side, I suspect, would absolutely consider it to be an issue of ethics.

your post is a bit clear, so let me clarify: are you attempting to define support for slavery or the KKK as an issue of 'ethics', but support for marriage discrimination as an issue of 'politics'? If so, I don't think that's valid.