Basically, my ManShep is going after Kaidan. I want to tell Cortez that "I'm waiting for the right man..." but that man isn't Steve. Can I do that and then turn him down gently, or will it lock me into a romance with him/offend him if I show no interest?
I'm not particularly familiar with the scene you're in, but it sounds like you're getting really close to Steve's "lock in" point. Based on your description of the scene and your options, I'm guessing that you haven't actually reached it but you're still not far from it.
The Mass Effect Wiki has this to say about locking in Steve Cortez as a romantic option.
From Mass Effect 3 Romance Mechanics, in the Romance article:
From Romance, in the Steve Cortez article:
Based on the flow of the article, and what you've said about the decision point you're on, I think you're probably still a few steps away from the commitment point. My guess would be that you can safely give either response without being immediately locked in.
Still, it's always safest to have a recent save (or autosave) point just before these interactions.