Besides the fact that having multiples, it takes time, money and energy. I've also thought that our small 3 bedroom ranch would be tight with 2 kids. No guarantee that they'd share a room for long or ever. DH is in school right now and he needs the 3rd bedroom for homework/his space... I seriously think if we added to our family we would need to move or add an edition to the house. Just thinking of other things people have to trade in/modify to add to their family. Do you think of the long term such as, "If we were able to stay in the first house we bought we could have it paid off by now." Also, "We could be retired by now." We have worked hard to get where we are (haven't we all?) and I don't want to make things even harder.
I don't know why I feel like I have to justify this decision, but that's the sort of thing that runs through my head... often!!