The professor's reasoning was that the Latin infinitive was a single word, not two as are English infinitives.

WW, you learned right. It was one of those Victorian thangs. I'll let my betters, tsuwm, Faldage and maverick (and who knows who-all else?), direct you to the right place but I think you'll find this in Pinkerton's The Language Instinct, which somebody cited somewhere in some forum (I'm getting too old to be participating in four bulletin boards... I can never remember who did what to whom).

And then again, you could always consult William Safire.