Wow! You have to make a decision. Will you stand by your tenants and help them fight this fight on behalf of people who love each other without regard for "boundaries"? If so, ask them how they'd like to proceed. Three against one is a good start. Standing up to bigotry is always the right thing to do, but you must make sure your tenants are comfortable with whatever course of action you (plural) decide upon.
The neighbor is just a garden variety bully, and the way to deal with them is with any means available, including media of all types. I'm not sure going to her employer is legally defensible, so I'd use the threat of it, if you thought it was helpful, before I'd actually do it. Also, is she a church goer? That's another avenue worth exploring.
My heart breaks for your tenants.
Sidebar: We have a house for rent. Two women want to join households and rent from us. One's application says the other is a "roommate", the other says she's a "friend". I told the agent to tell them we'd decide on the basis of their income, credit scores and references only. If they're partners, it's fine with us. It's none of my business if they sleep in one bedroom or two. Sheesh, who cares?