I wanted to respond to you because I hate when no one responds to someone who is putting themselves out there and wanting a response and no one replys. I didn't respond at first because I dont' know exactly what to tell you, I don't have much experience in your issue. But, I will say that you are right about what goes around comes around...and you might want to pick your battles carefully.
In Georgia where my divorce is, there is a thing called a "morality clause" in most divorces, basically it says that the parents cannot have members of the opposite sex spend the night under the same roof as their children unless they are married to each other. It prevents either one of you from having girlfriends or boyfriends spend the night so that the child doesn't have to witness this immoral behavior and stuff. It has backfired on many of my friends in many ways, one situation was that my friend went nuts when her XH took his girlfriend on summer vacation with them and they all stayed in the same room, then months later when she finally had a boyfriend she wanted to do the same thing and couldn't because she had charged him with contempt of court for doing it months before. So, she was stuck, which I felt was a good thing because the children don't need to have these people coming in and out of their lives or parents beds anyway, especially when you have only been dating them a month or two and it might not last. And, I have had other friends who's XH's have just married right away the person they were having the A with that broke up their marriage cause they didn't want to have to get their own place and "waste" money. I know I went off on a tangent, but my point is that sometimes when you "push" the rules of the visitation or divorce, it backfires on you altogether.
That is not to say that you shouldn't have boundries and stick to them (I am not good at this at all, if you have read my posts, I still sleep with my XH!!). But, life is not as long as you would think (most people say "life is short") and you need to remember that at this point you need to worry about you and your D4 and what is right for both of you. When the time comes that she starts to understand all this visitation stuff, then you might need to switch things up a bit and be more strict, but until then I would not worry to much about it. I would, however, have a problem with my kid around OM, I never allowed my kids around OW!! People who have affairs with married people are not people I want around my kids at all, they are immoral and I don't want my kids raised that way. My kids are older though and knew what was going on, your's is little so it will be harder to keep her away from OM. Good luck and post here as often as you want!
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Me-40 XH-44 T-21 M-18 Div-19 mo. D-18,S-15,D-11 Bomb-7/07 EA,PA Mvd out-9/07-to give me space mvd back-12/07 mvd out-7/08 back with OW since 2/08 OW broke it off-1/10 in and out of tunnel and our life since!!