I have to say that I thought the affair comment was a bit harsh as well. You have done a lot that is right but on this forum people tend to shoot from the hip and you don't always get to hear what you want.
I don't think you should necessarily close off your heart to L but in reality you may have to make a choice between your husband and him. Not everyone can cope with polyamory. My ex-husband couldn't. Over a long marriage (over 20 years) I fell in love a few times (there was something missing from marriage). We discussed an open-marriage but in the end he said all I really wanted him for was the financial support and friendship; it would be harder for him to find quality polyamorous significant others and so he wanted out.
I have to run but keep posting. I'm sure I speak for others when I say there is a lot of concern and support for you as well.