In that thread, I read that the OP's husband recruited the boyfriend and his former partner for wife swapping sex but due to problems in their relationship, the boyfriend split up with his former partner. The OP and the boyfriend hit it off anyway and the husband pretended to be okay about it but wasn't. Something that came to light later on. The combination of inexperience and the husband's revelation that he is struggling and may want to close the relationship at some point led both the OP and the boyfriemd to progress their relationship at a pace most would deem unwise. The boyfriend in particular became typically fixated on replicating the entanglements the OP has in her marriage, rather than focusing on developing their own entanglements, be they practical or otherwise. The OP wanted to maintain and progress with both relationships but the conflicting desires of her partners, particularly her husband who gave a deadline for closing their relationship, left her feeling stuck.
Now, boyfriend is freaking out about just how "secondary" he is because in his mind, her husband could non consensually replace him as the father of the child he helped to conceive, in every way that counts. Husband wants to have a "normal" family and will find it difficult to accept anyone being as entangled in their lives as they would be if they are a co-parent. And OP can't understand how it all went so wrong and just wants it to stop.