^ Exactly, this girl has made this whole situation benefit her. Not only does she get your 6k a month, but she gets to sleep with whoever else and get their money, too. You're getting screwed over.
Also, did it ever occur to you that beyond what she LOOKS LIKE your wife is a PERSON too? I'm sorry she's not Playmate of the Year, but I doubt you are either. She seems to have been forgotten in this whole situation....you obviously have little regard for her. It's selfish of you to carry out this behaviour and disrespect her behind her back because YOU don't feel like ending the marriage. Your wife deserves some happiness, too.