So called "morality clauses" are fairly standard in many lines of work, and most large companies simply won't employ you if you won't sign, as for them it's a requirement for employment.
That said, an employee does have some amount of legal recourse here, because as you stated, the terminology does tend to be rather vague, and from my understanding, for a specific portion/section/line of a contract to be legally binding, it needs to be both clearly defined and enforceable. In fact, that's why most employment contracts also have a line somewhere in there saying, "if any terms within this contract are deemed legally unenforceable or invalid, it does not render the rest of the otherwise legally binding contract null and void."
So morality clauses are indeed challengable in the court of law (as are competitor clauses that state you can't work for any company deemed a competitor, but that's another subject), and if one were to lawyer up, in general, if you didn't do something particularly grievous, you'll have a case as this tends to be difficult to prove(as typically the employer has to prove your actions caused purposeful damage to their image that created tangible financial harm). As such, most of these cases will be settled out of court so long as the fired employee can afford the legal means to do so.
