No thanks, I'm in a relationship

Should I ever be propositioned by an attractive lady (not the one with whom I cohabitate), I plan on using the above phrase to decline.

I would then follow up with an overview of Bob's relationship paradigm for identity. I expect that a 10-15 minute powerpoint presentation discussing the subtleties of relationship types, actors, and life cycle would both dampen any remnant ardor (I know it does for myself) and mitigate any embarassment the lady would be feeling at the rejection.

I do feel it's best to prepare for all eventualities - even those with no historical precedent.