Non-issue.
Businesses can chose not to serve anyone, as long as it's not BECAUSE they are a protected class. You can choose not to serve a protected class, it just has to be for some other reason.
The recourse is a civil suit, but that won't succeed unless there are legal grounds, and the pax would have to prove the airline banned them for being gay, or brown or something legally protected. The airline actually doesn't have to "prove" they had sufficient cause, they can chose not serve you for almost any reason they like.
And they have to do something about it, if they let pax get away with it it will just encourage more of the same and if the crews feel like the airlines don't have their back it's going to be diversion city at the first hint of trouble $$$$.