A date doesn't mean you're committed to a monogamous vanilla relationship - it's simply a few hours out of your life to get to know someone. If the person in question is in some way offensive to you that's one thing, but if you think they may be a nice person then I wouldn't turn them down based simply on the assumption that they are or are not a certain way. How do you know they're vanilla? And even if they are, how do you know you still wouldn't like to have them as just a friend or an occasional coffee date?

If you're uncomfortable calling it a "date" for fear of giving the wrong impression (i.e. that you're sexually attracted to them or want a committed relationship), then there's certainly nothing wrong with saying, "just so we're clear I'm not ready to date or be in a relationship right now, but if you're interested in getting to know each other over lunch that would be nice."

The majority of my friends aren't privy to my sub side, and that's perfectly okay - I still am glad I know them and have them as friends. Those that are "in the know" were introduced to that side of me over time. Same thing with lovers - over time as trust was built. J and I didn't start out as a D/s couple, we evolved to where we are and are still growing. Thank goodness for that, had either one of us dismissed the other right off the bat because we assumed we wouldn't find what we were looking for we would have missed out on something spectacular.