The issue as to whether it is a sin or not is a question of religious values. Regardless of where one stands on this, there are secular (sociological and financial) reasons as to why "gay marriage" and other "gay rights" proposals are harmful to both civil society and government in general.
As to the statement that "homosexuality breeds like cancer," Ms. Knox is wrong as you said, wrong if you take the word "breeds" either literally or figuratively.
But the First Amendment doesn't necessarily exclude the right to speech that is factually wrong. Heck, leftists have been factually wrong on so many issues for so long, yet no one questions their First Amendment rights.
It's nice to know that there is a clear Supreme Court precedent which appears to support Ms. Knox' position that it would be unconstitutional for the school district to fire her in retaliation for her Facebook post.
Thanks for the ping/post; thread. Thanks to all posters. Tremendous insight into Chris Christie.