So glad you got S to open up and examine his emotions, and you two seem to be getting in the habit of talking things over asap instead of him holding things in until they can't help but erupt in hurtful confusing ways! I hope this continues, I hope you two get more time together soon, and I hope you increase your sex and kissing to increase the warm bonding hormones a good relationship should have.
I had a thought about the actual poly bit though, when I read your OP. You say S wants you to get a lover, but he doesn't want one for himself. You don't say WHY he wants you to get a lover. You didn't seem to want one, and have just gone along with it to please S.
However, now you've gotten accustomed to it, and have had some enjoyable experiences with a few men.
I get the feeling S has a desire to feel cuckolded. The idea of another guy fucking his wife makes him turned-on, yes? However, I also get the feeling he feels threatened when it actually happens, especially because you need a guy you really like, before sexing him, whereas what S wanted was just for you to have a FB, sex alone.
Now, it's normal for guys to get off on the idea of their female partners getting fucked by another guy. (Read the great book Sex at Dawn if you don't believe me. It discusses the polyamorous or promiscuous behaviors of our hunter-gatherer ancestors.) But it's also normal for people to feel threatened when their partner actually develops strong feelings for another partner. Jealousy and fear of loss develops. This is so common. New Relationship Energy can overwhelm the new lovers and make the primary feel quite neglected.
So. You say you and S haven't had much sex lately. Is it possible however, that when you are on a date, and having sex with someone else, S is masturbating to the idea? Many men get all turned on by something, but once they cum, they are then disgusted at the fantasy (or even a real person, if they were actually fucking) that turned them on seconds ago... That's a guy thing.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve.
~Percy Byshe Shelley