How should I reply to a guy that asked me if I'm flexible?

I'm on an app to make new friends, I started talking to a guy a few days ago. We flirt a little but I'm no where near trying to hook up with anyone or get into a relationship. He initiates the flirting every time, today he sent me a message asking me if I'm flexible. I'm a little offended by the question as to it is kind of a sexual question & I'm not sure how to reply... any suggestions? I'm not trying to cut him off as a friend, we do have normal friendly conversations other than the times he starts flirting with me but I don't appreciate the sexual passive questions.
How should I reply to a guy that asked me if I'm flexible?
Post Opinion