Whether the donations are public or private, they are still something which one could reasonably expect to cause outrage and draw protest. It is the effect of these donations that people find offensive and dangerous, not the manner in which they were made, so whether public or private, many people are outraged.
In any event, Dan Cathy publicly took credit for making them on radio, which started off this whole brouhaha, so yes, they did make a public statement.
How is it ludicrous that you bear responsibility for the actions of groups you fund (not just support)?
If i give money to a group which supports tax reform and persecution of read heads ostensibly with a view to supporting their tax policies, I have no personal responsibility if the money I donated is used to persecute red heads?
That my friend would be ludicrous.
Anyway, as I pointed out, Exodus International (for one, being the only one I googled) is not a multi-issue group, so it's kind of irrelevant in this context, isn't it? And it should be noted - these groups aren't just opposed to marriage equality, they are generally anti-gay full stop. Some of them (or their representatives) have supported (directly or indirectly, depending on reports) Uganda's attempts to introduce the death penalty for homosexuality, called for criminalisation homosexuality in the States, and promoted harmful "gay cure therapies."
Why shouldn't you link race discrimination and discrimination based on sexual orientation. The principles are the same - equality. If you believe in equality you believe that all forms of discrimination against any minority, for whatever reason (or none) is equally wrong.
Moreover, if one group who feels discriminated is lawfully and "morally" entitled to pursue a particular tactic to protest what they perceive to be discrimination and prejudice, objectively speaking, an other group should also be entitled to pursue the same tactic.
Unless you are suggesting (which I would like to think you are not) that the legitimacy of a protest action and/or the entitlement to protest is to be judged on the merits of the cause? Though of course in the 60's, many doubted the merits of the civil rights movements, and so the restaurant sit-ins would likely have failed any test of its merits.
Lastly, the right to protest is very much part of, rather than being distinct from, the right to free speech. A protest is making a public statement - that's all. "Free speech" covers everything from art, public protests, poems, speeches, and clothing. It is generally only curtailed in a democratic free society were it used to infringe upon other peoples rights.
The protesters didn't interfere with anybody's legal rights - they voiced their opposition and hoped to persuade others of the righteousness of their cause. People were however free to continue to eat there - as evidenced by the crowds attending to show their support.
No, feel free to say you disagree with them, either on the issue of gay rights and equality, the decision to protest, or the manner in which the protest was conducted. You can also say that it ended up doing more harm than good.
But they most certainly have a right to do so, whether or not you agree.