Funnily enough, I walked from one this morning. It wasn't a b & s, and her profile pics weren't that misleading, but I knew from the word go I just didn't fancy her.
It can't be good for a girl's self esteem to be turned down in those circumstances, so I said my stock excuse, "I'm terribly sorry about this, but I haven't done this before, and I'm afraid I've got cold feet, and I can't go through with it, sorry."
Whether they ever believe it I wouldn't know, but it's just an effort to try to leave us both with a bit of dignity intact.