Hmm, well, I I am actually feeling much less sympathetic towards your husband now.
On the one hand, this -- "Why was getting a piece of ass more important than holding me and telling me everything was going to be ok." -- is still unfair. You said above that he didn't want to go with you because he couldn't handle it emotionally because of his mom, and you said you believed that. You could be angry at him for not trying harder to overcome his sadness about his moms to support you, but if you really do think that's what his decision was about then he didn't stay just in order to get a piece of ass and making it about that is unfair.
On the other hand, it sounds like he's being very, very insensitive. I also I think he may be lying, on an emotional level. Look at it this way... either A) he was lying when he said he was in love with two women (why would he lie about that?) and he really is just after a "piece of strange"... in which case it's pretty callous of him to be pushing you so hard when you're having so much trouble, or B) he was telling the truth when he said he's in love with more than one woman, and that's why he's pushing so hard for this now because it really does matter to him. Chances are good he's confused himself. But I thank you may need to confront him with these two scenarios and find out for real which it is. If he *does* have feelings for her and is lying about it, which I think is more likely, chances are good he won't come clean if he thinks that doing so will break your heart. So... *would* it beak your heart, would you be furious with him, if it turns out he loves her? If not, you have to help him believe that. If so, you have to be honest about that but tell him that you need the truth anyway, because being lied to would be worse, and even if he tells you something you don't want to hear you will respect him for it and deal. That's one approach, anyway.
It's also really messed up that he won't give you the post-threesome one-on-one reassurance that you're asking for. It doesn't need to make sense to him, even though I think it makes perfect sense for the record, he can just accept that you need it and your needs are important. What will it hurt him to help you with this???
Bottom line, all the other questions and complications in this situation aside, he REALLY needs to work on his empathy to be a better partner for you. It's like they say, poly really shows you the things that need strengthening in any relationship.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.