YANBU and it is indeed horrible when someone asks insensitive questions.
I speak as someone who has had 5 miscarriages and a lot of treatment to have DS, now 10 months.
But I have to say i feel that most people don't ask maliciously and are just unaware that they need to consider whether people are having problems or not. I have asked in the past too and although now I know not to, previously it was just a case of making conversation and getting to know someone.
When people asked me, when going through ttc and recurrent miscarriage treatment I told them. Not in a way they made them feel bad for asking but just in a way that carried on the conversation. Of course I know not to outright ask now but if you can tell someone wants to talk about it or bring it up themselves I will happily have a conversation about it.
When I was heavily pregnant a conversation like this started and it turned out the lady I was speaking to had also had miscarriages. I told her about my treatment and we swapped email addresses and, to cut a long story short, she had the treatment too and now has a three month old daughter.
Like I said, I know not to ask now whether people want children or not unless they are close friends but sometimes those of us who are having problems do in fact want to talk about it. Although I realise some people are a lot more private and can't, understandably, face the pain of talking about their struggles, if more of us are open about our problems maybe that's not such a bad thing. Children are a fact of life, whilst I realise it's not the same (as said up thread) as choosing to buy a car or a house, babies are born every minute of the day and as such are going to occasionally pop up in conversation.