So many good posts in this thread already, and so much good advice.
Your primary goal has to be to do the right thing for yourself. Whatever happens to your other relationships you have to live with yourself. Do you see yourself as an honest and ethical person? Does it bother you when you don't live up to that? If the answers to both of these are "yes" then you need to do what is necessary to be that person again, no matter the consequences. We are all human, we all make mistakes. The key is to recognise them, own them, and make amends for slip-ups and put things in place to make sure that this doesn't happen again.
Assuming that this is important to you, the next important thing in your life is the relationship with someone with whom you made some very solemn promises - your husband. Without acting in a trustworthy way towards him you are putting your promises (and therefore your marriage) in some severe jeopardy. Take care of that next. Deal with whatever consequences are going to come from this, knowing that at least you owned your stuff and lived by your own code of ethics.
Then you can start thinking more about the poly thing. Talk with your husband about it - do some discussions about what he does, and doesn't want to know about. Don't do it in the context of a specific third person, but as an abstract. See if the two of you can come to an agreement about boundaries. If you can, then you are free to explore things in ways where you know that you are living up to your revised wedding vows, and are behaving ethically towards your current relationships.
L's issue with his partner are for him to deal with - you didn't take advantage of him while he was incapacitated - this has been building for a while - he has to own his stuff. As a courtesy you can let him know that you will be telling your husband about what happened - maybe the two of them can have a private talk about it once you have cleared things up between you and your hubby.
But to me that is all future - you have to make sure that the things that are your priorities are straight.
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