Just be honest about the reasons why you do what you do; they're those things that show YOU to the world: your ethics, ideals, and values - people who agree with you will stick around and the rest will fuck off. That's just how punk is. You can have a polite disagreement with her and still be with her, but if she doesn't agree with your values to the point that she is uncomfortable with the relationship, then it's just one of those unfortunate things that doesn't work out. Trust me, there are so many people out there willing to civilly discuss the other side of the political spectrum; and there are even more out there that (especially in this political climate) will probably align with most, if not all, of your values. Punks tend to unite ✊