I just would like to add that all this intensity (the 'mushy' stuff you refer to) passes - or at least dies down in phases only to revive. It's a normal part of a mature relationship. I'd urge you not to misread what is a perfectly normal evolution in your relationship.
That being said, there are a lot of things you can do to try to restore some of that. Doing special things together that remind you why you connected with each other in the first place can be a huge help. I've seen (even in my own) relationships that drift, refocus, come back together even stronger, albeit in maybe a slightly different way. But these 'different' ways can often be better, healthier !
We're a society of adrenaline junkies and that often applies to relationships. We want for that high intensity experience (often referred to as the NRE phase) to last forever. But that's really unrealistic. Life, and relationships are simply not like that ! Don't let that disillusionment overshadow something that may otherwise be very beautiful
This really has nothing to do with 'poly' yet, except for the substitution of some seeming lost intensity. Just acknowledge it for what it is and don't read more into it. You need some additional intensity ? That's ok ! You may even be able to find this together ! That's what (given what you wrote initially) I would strive for. Something you can SHARE, that will pull you back together.
Then you have a win/win for everyone including the children
Try it you may like it