My personal take is, it's very different saying "I'm against murder", and saying "that man is a murderer". We have libel laws to protect individuals from slander, and I think that's a good thing. But I don't think there should be any prohibition on talking about policy in general.
So it should be legal and tolerated for people to loudly proclaim "all white people are racist", but not put up a billboard of some white dude, and claim he is a racist, unless you're prepared to defend that allegation in court.
Because again, that is exactly the rhetoric we're discussing: Rhetoric which says 'X is mentally ill, grooming children for sexual activity, and should be executed'.
If it's okay to say it when X is an LGBT person, or all LGBT people, then it is also okay to say it when X is you. So why try to sue for something you are okay with?
Conversely, if it is inappropriate censorship to try to moderate such messaging when X is an LGBT person, or all LGBT people, then it is also inappropriate censorship to try to moderate such messaging when X is you.
What if they change the law to say that hate speech against ethnic minorities is legal?
Taking the current law, at any given moment, as our standard for ethics is not a tenable position.
After all, no line means no line, right? That's what the minorities in question are being subjected to, that's the sort of rhetoric we're discussing. Indeed, such a billboard would be as much about "protecting the children" as the example OP gave.