A few more things before sleep...
Thanks to everyone who has taken the time to reply. You all have helped.
KMS, great post, you probably made me feel the best about this situation among anyone here. In fact, Al offered to break up with his girlfriend for Sue shortly after Sue and I started...the word "dating" might be a bit strong, but she knew that I liked her at that point. She turned him down because she wanted to see how things went with me. (The jealousy kicks in a little bit, because I think she might have actually wanted to be with him, but not willing to be the homewrecker... but that might be a little paranoid.)
Also, just to clear a few things up
--Al's girlfriend would believe me if I told her, and my girlfriend wouldn't lie to her either if asked about it (I'm still probably not going to tell her, but I haven't completely made up my mind). I'm somewhat surprised that everyone is so quick to tell me not to tell her. If someone knew my girlfriend was cheating on me and let me continue the relationship withought that knowledge, I'm not sure I would ever be able to forgive them.
--I have no double standard about male and female sexual partners numbers.