I guess I'm a bit more liberal on this because I have always gone out to lunch with male co-workers, vendors, etc. I wouldn't consider it dating at all. Maybe I'm naive.
Sooo, is dating ok? Not right now. Is meeting up with a "friend" here and there ok? Probably. If you can trust yourself to keep it platonic. Certainly seeing the same person over and over for lunch is getting on that "slippery slope" that Shirley Glass talks about.
There's that fine line you just can't cross. And, personally, I am now "hyper-aware" (I think I just made that word up) of crossing that line.