I think this went way to fast, especially as you have a child. I find it astonishing that you would think this was going to be okay for your child. Children get attached and don't understand adult stuff. Of course your child would think that these people would be in your life forever.
That being said, there is still a way to handle it as far as I'm concerned...
It think if this were me, I would start slowly backing away. It sounds like you have allowed NRE to take over your life in the last months and have made some major botch ups because of it. As far as I am concerned you should be staying at home more, picking your own child up from school, and eating at home with your child more often.
I am not sure why you thought it was a good idea to date the woman when it was the man that you loved. It wasn't mandatory that you date her, just because you fell in love with the man. You could of worked something out to just date him I think.... no wonder she has been doubtful, she has every right to be, it sounds like you don't love her, you love him and she came along for the ride.
Ya, this went waaaay to fast... back up and give them some space and yourself. Most of the drama will go away I suspect when they get their marriage on track by you not being there all the time. They need to connect again and you need to get your life back again... then see where you are at with continuing the relationship you have with the man and perhaps continuing with the FRIENDSHIP you have with the wife
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