I'm very sorry you're new triad is experiencing this. I am also in a MFF triad with my husband and girlfriend. Our relationship is about a year old and we've had quite a few discussions about more children. We currently have one child, who was born prior to our current relationship.
Having a child with someone is not a decision to be taken lightly! As much as I love my girlfriend and husband, I get panicked whenever we seriously discuss her having a baby. I know that I want to expand my family, and I know I want her to be part of it, but I'm not sure the relationship is mature enough yet. Its tempting to think 'it will be easier because she does the hard pregnancy and breastfeeding work' but that is nonsense! You, as a stay at home partner, will be her number one support. Are you expected to continue to be stay at home parent? Is that something you desire? What about when you want more biological kids of your own? How will she feel about that? The list of questions goes on and on!!
All those are internal questions. Sometimes I think that stuff is the easy stuff when I start to contemplate how we will present ourselves to the world. A baby brings everything to light. There must be disclosure to all three sets of grandparents. There will be constant questions about 'whos baby is it??' and so on. Then there are the side remarks of people who think you (as his wife) have been played. All these things can have deep emotional impact on a relationship. Again, is the relationship mature enough for this?
Finally, there is the fact that adding a child to the family is adding a new person. But by entering into a relationship with her, you've JUST added a new person. Why not explore each other? Learn more about each other? Would you have a baby then have another one 2 months later? Of course not, because every one needs time to adjust and get to know the new family member. The new family member needs time to feel part of the family.
I urge your family to take time. Love and desire for children will still be there in a year or two. Given how young you all are, you have plenty of time to enjoy yourselves and really build trust before taking what is arguably the biggest step you can take in a relationship. Good luck.