I am not going to respond to everything else that you and Iisjustme have said, as we are all smart of enough to know that it is disingenuous to claim that Mr. Cathy was not expressing an opinion on the same-sex marriage debate.
As for your question above, I agree with you that the hostility towards Mr. Cathy is misplaced. He expressed an opinion, and one that any intelligent and honest person already knew he held. It's not like some liberal who loves his fried chicken sandwiches had any reason to be shocked to find out that the management of CFA disagrees with them on some important issues. And, one thing that liberals have strayed from (and one reason I distance myself from them more than I used to) is the idea that basic civil liberties are of utmost importance.
In the case of same sex marriage, you have an arguable case that there is a civil liberty issue at play. But, when a person expresses an opinion and is told by those in power, "you are not welcome in our town because of what you believe", you have a clear cut civil rights issue. And, sadly, the liberals on this website are divided on that point. And, that is where I have a problem. Whether I agree with him or not, Mr. Cathy is entitled to express his opinion without having his business interests threatened by government because it is an unpopular.
A Muslim who expresses the same view should have the same right to do so. But, you are right, the outrage would not be nearly as intense (if it was at all) from the left. Ironically, the outrage would come from the right instead. But, I can't speak for either side, because I disagree with most people on both sides: I am fine with civil same-sex marriage -- I don't oppose it -- I tend not to get too involved in the pro-SSM movement either. I am much more concerned with the more established civil rights, such as freedom of speech, religion, and from undue invasions of privacy.