only child

Alex • Blessed to be mom to our miracle IUGR baby boy! I've never met someone as strong and brave as he is!
Due to the diagnosis of severe iugr I've been on hospitalized bed rest for 3 weeks now and will be here til our son arrives. He will be born premature, basically it's only just a matter of time before he goes in distress and needs to be taken out to live in the nicu for a little while.
He's so strong and a miracle already. But the topic of whether we want another kid has come up, and I worry about another kid having to struggle like our son is right now - it doesn't seem right.
When I tell others that our son might be our perfect one and only child, people laugh and say "oh you'll change your mind." 
Why can't it be acceptable to just have one kid?