It seems you're still dwelling on the negative and the what-ifs. Being online at the dating sites is not a crime. Like you, he's looking for love. Although, at that time he had not separated from his wife it would seem right? Yes, you absolutely can forget both Stacy and Frank, or at least push the image of them out of your mind and replace it with a positive and happy one. When you meet w/ Dave, you may casually ask him what had prompted him to start the online dating, what had he found helpful or unhelpful through his experience w. it, etc. It seems that he may have also been burned out because of people posting old pictures of themselves and misrepresenting themselves. That should at least make both of you laugh.
And Judy, remember that you learn from your experiences. They give you a frame of reference in future situations. How else would you have realized that Stacey was not good for you and that you need something way different from a relationship?