It seems to me this is the free market in action. Your labor is worth so much less as a bigot that you are unemployable. Tough shit.
Unless you want to add political beliefs to "protected classes". But it was my understanding that conservatives don't think the government should be involved in mandating voluntary associations.
And the email did not say to resign if you supported Trump, it said to resign if you would engage in bigoted acts. "demeaning, insulting and ridiculing minorities, immigrants and the physically/mentally disabled" is how those acts were specified. And frankly, if that's how you're going to behave, then you have no place in any organization.
Pertinent parts of the email:
"... I absolutely reject the nationalist, anti-immigrant and hateful politics of Donald Trump and will work to shield our community from this movement as best as I can......If you do not agree with this statement then please reply to this email with your resignation because you have no place here".