I never know how to handle this question.
First of all, that is none of your business. I don't care if you're my best friend, a colleague, or an ex-boyfriend. My love life is no one's business by my own, so all of you busy-bodies should just keep to yourselves.
But what is the definition of "seeing" someone anyway? It seems like everyone has their own idea on this subject. Some feel like it's as soon as you've gone on one date; some say it's after a couple of dates; some say it's the same thing as calling him your boyfriend. Is "seeing" the same thing as "dating"? The nuance is killing me. I feel like we need to just cut to the chase and ask people, "Are you having sexual intercourse with a specific person on a regular basis?" Who are we kidding? People want the dirty details (well, most do. I prefer to keep my own tales of conquest to myself, and I really don't care about yours). Stop pussy-footing around and just ask the question that you really want the answer to!