An Enlightened Master is ideal only if your goal is to become a Benighted Slave. -- Robert Anton Wilson
While it is completely within their right to do this, it is also within the rights of their staff to leave and their students to find other schools. I know this is going to come out as mean, but I honestly hope this hurts them enough that they either have to change their policies back or close. It would suck for those students who really want to learn there, but it is likely that they would get a better education somewhere more tolerant anyway.
The world is changing to a more tolerant place, particularly of homosexuality and premarital sex. Those unwilling to change with it or worse, take steps back to being more intolerance, are likely to find themselves left behind.
"A woman is like a teabag, you never know how strong she is until she gets in hot water." - Eleanor Roosevelt
Keep your religion out of other people's marriages.
Yes they have the right to discriminate. It's stupid to do so. It's even more stupid to do so after the fact. You're getting rid of people who have been loyal to your institution and integral to its operation. If they don't fold, it will be shocking. Finding qualified candidates to fill the positions before next term is going to be a rush. They'll have to contract and in private education, that's a death knell.
Im not actually sure that they are within their rights to do this. As far as I know you cannot fire people for doing legal activities on their off time.