If it happens repeatedly in a forum, like facebook, where his students eventually find out about it, yes. (and with 700 friends they WILL find out about it, as it’s far from private at that point, regardless of “privacy settings”) How would a gay student in his classroom feel knowing his or her teacher is so disgusted by them that they almost throw up?
If he just said “I don’t personally agree with gay marriage,” or “I believe homosexuality is a sin,” it would be a different story. The way he expressed his opinion was rude, hateful, and vulgar. Additionally, he could have expressed it in his church or sitting around with a group of friends, but instead he chose a public form where tons of people could read it and where it never goes away (even if you delete things on the internet, it only takes one person taking a screen shot for it to live on forever).
Imagine the converse situation where a teacher said that Christians make them want to throw up and that churches are cess pools. How would a Christian student feel in that classroom? How would a parent of a Christian child feel about sending their child into that classroom?
Tons of employers and schools demand that students or prospective employees add them on facebook (getting around any privacy setting you set up) and might then fire you or not hire you because of what they see. I fail to see how being a state employee makes him exempt from the same scrutiny I have to live up to.