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:
Example "locked-in" locations are:
- Steve Cortez: Kissing him when dancing at Purgatory after convincing him to take some shore leave on the Citadel.
From Romance, in the Steve Cortez article:
Eventually, Cortez will be in the Purgatory Bar. Shepard notes that Steve looks happier and Cortez replies that Shepard was right about the past being his, and that no moment should be taken for granted. When the Commander expresses doubt that a nightclub is to Cortez's taste, Steve explains that there was an energy and life in dancing, suggesting that the eye candy isn't too shabby for him either. Shepard wonders why Cortez isn't looking at him, though Cortez admits his interest and invites him to dance. While Cortez and Shepard dance, the Commander tells Cortez about his feelings, wanting to be more than friends. In reply, Cortez thought he felt there was something between them but was merely pessimistic about it. Shepard affirms that feeling and kisses Cortez.
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.
I know this is late, but just in case someone else stumbles on this and wants a straight answer.
You can say okay to dancing with him without locking into the romance.
They dance a bit and talk, and he says something along the lines of "You're a good friend". Then you get 2 options - "You're a good friend, too" or "I want to be more than friends". That's where you lock in if you choose to.