I'm going to yell at my roommate for asking my girlfriend about her sexual preferences
Why do you want to yell at him about him asking your girlfriend questions?
I don't like my blindsiding roommate and I feel like he acts on the level of a 3-year-old.
Even if he had not blindsided you, it was not okay for him to ask her questions about her sexuality. It is okay to be mad at him and to tell him so.
I should stand up for my girlfriend.
I agree, be firm but also smart.   Do it in a way that avoid physical confrontation and allows you two to still get along after.