Reading your story, I get the sense that your husband is just completely wrapped up in his own emotions and isn't being very considerate of your feelings. He wanted to be able to bang a whole bunch of other random women, and was okay with you taking your friendship to another level while he was busy getting laid. But now that he's getting bored of meaningless sex, now his jealousy is coming to the surface.
Some people get way more jealous about emotions than sex or vice-versa, and try to make all kinds of rules and restrictions accordingly, as if feelings are so easy to control.
Marriage is supposed to be about compromise. If I were in your shoes, I'd be willing to give him some time to get a grip on his jealousy, give him links to articles to read, be willing to talk about whatever his insecurities are, but would not be willing to return to monogamy permanently because of his feelings. He's only one person, there's no reason that his feelings are more important than your feelings or your friend's feelings.
Also, you mentioned that it's not romantic...Too often, people think intimate=romantic=deep=meaningful, like it's got to be all of those things or none of them. An intimate/deep/meaningful relationship can feel threatening even though it's not sexual or romantic, and all of that should be discussed- possibly with all 3 people in the discussion.
If you try to understand why your husband feels this way, that will deepen your relationship. If you let him restrict your freedom without understanding why, that will lead to resentment, and create a gap between you.