Ari - I was hoping you'd post on this since you've recently experienced being poly with a new baby. Maybe you can give the OP some inside to how pregnancy and multiple partners worked in your family?
So, and I am not sure I announced this, but both my wife and my gf have given birth recently. My wife a little over 2 months ago, and my gf .. a few days ago. For those counting, that's 1 2 month old, 1 newborn, 1 2.5 year old, and a 6 year old
Life changes so completely but it can just work. You do end up in some ways pulling back to your original coupling, simply by default, mommy is number 1 and dad runs around trying to help. All the others can do is support where they can in the time they have with other kids and babies. Managing a house with 4 kids in 3 different age categories is a really tough thing.
You have to focus on the baby, and in all honesty the baby doesn't want anyone but mommy (or her boob to be more specific) and the amount of work that goes into support the process of
eat
feed mom
help her sleep
baby poop
baby change
eat
sleep
etc
etc
Is incredible. Its a lot of work for a family. As long as you understand this is all bonding time, you can easily help and contribute. At this stage its all about the baby and momma.
Honestly in my case though, the relationships themselves have .. slowed.. its hard to quantify and it isn't a poly thing, its a relationship thing. The babies take a lot of energy. The poly family dynamic helps that, so we can do what we can to simply relief pressure.
A couple of months in, sleeping is better, sex drives increase (or the energy to take advantage increase), and things start to roll into a normal state (notice I didn't say back to normal).
I know for us we could never get involved with people who didn't understand the above.. being parents change the entire world, and people either can fit into that world, or leave. Friends, family everyone has to learn to adjust their expectations of what I, we, all of us can offer.
Its a wonderful experience, don't exclude yourself by default. I am a dad to 4 kids.. father to 1. You can engrain yourself in their lives by helping during these initial stages. Being pregnant can be a tonne of work too, both of the pregnancies in my family had difficulties, imagine two bed rested women.. Myself and my metamour have been running our assess off for 6 months straight.
You are also going to feel like the dad (thats a phrase from gf during my wifes first couple of months, she finally understood what is was like to be the helpless dad.. we really have nothing on moms.. when that baby cries.. ya.. ). You don't get the mommy bonding, you don't give birth, but you can help, wherever there is help to give.
Good luck.. its a tough choice, do you want to be a parent.. first and then a lover or would you rather be a co primary? Everyone, forever will be and should be secondary to the kids.
BTW did I mention how much work and stress it is. I have had pets of all kinds, I have have uncled, I have been the god father.. etc. I have worked multiple jobs (all in the very high stress category).. I have done it all at the same time.. being a parent is both incredible and quite literally the most difficult stressful thing I have ever done.
ps ignore the mom. Communicate with the couple.. Mom has nothing to do with this.
PSS I have no idea why traditional couples would want kids.. and single parents.. my hats off to you.. thats just insane..