Curvygrace: What has worked best for me in the past is to simply say "I was single when I met you. I had an emotional need that was unfilled, and thus I was open to a romantic relationship with you, a client, who I really liked. The same goes for any classmates, friends, whatever. If I'm single, I have an unfilled emotional need and will be open to a romantic relationship. If I'm in a healthy relationship with someone I have chemistry with, I will not have an emotional need. Thus, I will no longer be open to romantic or emotionally involved relationships with clients, colleagues, classmates, friends or strangers."
Makes sense, no?