Since religious belief is constitutionally protected, this becomes a rather sticky point.
One could argue, rather spuriously, that a Professor in a medical school that publicly stated that he would not give a homosexual a recommendation because the lifestyle is not healthy nor conducive to a career in healing, would be similarly at risk for a lawsuit. While not constitutionally protected, many would feel that the Professor SHOULD be sued, that such criteria, while having some legitimate value, blatantly discriminates.
Let's say the original Professor in question discovers that his student attends a nearby fundamentalist Christian church. Does he disqualify him on that knowledge? What about other religions that believe in Creation - Jews, Muslims, and Hindus all have the universe being created by a Devine Being. If you practice any of those religions, are you automatically disqualified to be a physicist in this Professor's view.
Another point to consider is how important is this recommendation to their future job prospects?
I guess my point is if the "left" is going to bring silly discrimination lawsuits for barely tenable reasons, why should we be surprised if the "right" begins to bring their own?
Since religious belief is constitutionally protected, this becomes a rather sticky point.
I don't see it as a sticky point at all. The relevant Constitutional question here is the professor's First Amendment rights. The right to speak freely comprehends a right not to speak. His reasons for not speaking are immaterial, let them be ever so bigoted. The court cannot compel him to speak when he chooses not to.