My guess is that no one will engage in a constitutional discussion. Some in the crowd would stomp out the burning pride flag and then a few of them would go on to attempt to stomp on those who lit it.

Why are people afraid to say this?

No one's afraid to say this - several times upthread I noted that the crowd would detest the message inherent in burning the pride flag. Whether you think that manifests in angry remonstrances to the local paper and a meditation circle, or people "stomping," depends on whether you want to indulge a stereotype of liberals as ineffectual pencil-neck weenies or violent agitators.

In reality, the liberal crowd would have the same reaction as a conservative crowd would have if you burned the flags in the reverse order - acceptance of burning a Pride Flag as a legitimate expression of free speech, and outrage (perhaps to the point of "stomping") at the burning of the American flag.

We bring up the constitutional discussion because Dope made a point of including that detail in his original hypothetical. In his hypothetical, the demonstrator specifically told the crowd that the burning of the American flag was a demonstration of free speech, and that the crowd expressed understanding and agreement of that. When construing hypotheticals, the general rule is that the deliberate inclusion of a specific detail is intended to have operative significance in the hypothetical. So we proceed in the discussion to presume that the constitutional discussion was not irrelevant (else why mention it), but instead is something that the crowd has actually acknowledged. I agree that's not very likely, but it's not my hypothetical....