The professor's irrational hatred of Bender and Amy's human-robot relationship was completely different from his earlier reaction to Fry and Lucy-Liubot's relationship in an earlier season.
I always thought of it as Lucy-luibot Fry dated wasn't an actual robot per say, but just a machine built to love him and nothing else. While on the other hand, Bender is an actual Robot, who could think and make decisions.
So I think his hatred came from the fact that both of them had a choice in the matter, while Fry and Lucy-Luibot, only one of them had the choice.
That's just my interpretation
