A friend invites you to something you disagree with - what do you do?

How would you respond? This is inspired by an "RSVP rejection" letter I saw on twitter which reminded me of one a roommate of mine wrote. (I'd never encountered the idea before)

Examples: Friend's 50th wedding anniversary, but the friend (and their family) voted for the other side in the last presidential election. 

Your coworker is retiring! But they're having their party at a business you don't want to patronize. (a strip club. Or a BBQ place, but you're vegan. Or a company that donates profits to a cause you disagree with.) 

A wedding you don't "agree" with - you don't like their partner or you are opposed to the type of marriage. (e.g. polygamous, giant age gap, some sort of self-actualizing "marrying myself" sort of thing)