The supreme court explicitly held that people have the constitutional right to hold an anti-gay rally outside of military members' funerals [1]. I think that the vast majority of people (including the overwhelming majority of people holding anti-gay views) would find this behavior atrocious and immoral. But it's protected free speech.
The problem though is that enough of this kind of stuff happens and laws begin to change. Sure, it's one crazy group in this case, but if this sort of behavior were prevalent enough, peoples' views on free speech would change. Sure, this is an extreme example, since something specifically mentioned in the constitution is very hard to change. But there are plenty of examples of laws that only exist in response to some idiot(s) who decided to ruin it for everyone else (as a very simple example, I no longer have the right to buy more than one pack of Sudafed at a time where I live... they didn't just dream that law up out of thin air).
[1] https://en.wikipedia.org/wiki/Snyder_v._Phelps#Alito's_disse...