I have come across this situation, and for me, here's where it's at:
I'm not looking for something on the side, and I won't enter into any relationship I have to hide from Fidelio. When I meet someone who IS looking for clandestine adventure, I know we're not looking for the same thing, and thus simply not going to be compatible. Same as if he/she were a heavy smoker, active addict, or pro-ana. So it's "thanks for the interest, but it's a no-go." No harm, no foul, and move on.