Sometimes people are looking for a bit of happiness in thus hard world and this could be their way of looking for something to be happy over. A new marriage or a baby can be just that...new hope. I try to be sensitive to these things and try to be respectful of others, remembering getting those questions myself, but sometimes I forget.
It can be hard, but you know, people will always ask. Don't be rude, but change the subject, turn the spotlight on them with a return question or walk away. There is nothing wrong with that. If someone can handle the truth send you can trust them with it, share it. You might never know if they are going through similar and having some hard time with something and you can help support them.
When I was younger I got those questions. When I got married at a later age and was expecting shortly after, we were questioned about not waiting for a few years first. Really? When we could only have one child, we were questioned. It was especially hard then and also after a miscarriage too. When we finally were blessed with more kids than the norm, we got questions and sometimes harsh judgment about having so many, but then there were a few old souls (often strangers) who encouraged us on hard days. Those were truly beautiful people.
We were going to adopt at one point but were never so privileged. I remember getting attacked by one lady I just met and she was harsh. My dh left and there it was. She hated adoption and felt like she should cut me down. I certainly felt hurt, but sad for her that she was so angry. Hopefully she did not do that to too many others.
It is your life. It will go the way it is supposed to with happiness, sadness, disappointments and pleasant surprises. Nobody can truly plan their course and nobody can plan yours. It is nobody's business, that is true. But remember as others have said...questions & judgment will happen at every stage of life. It will always exist.
Live your life, make it full. Be positive. Don't let these things/questions/people bring you down.