Because to them, getting married and having kids to them is the way to live. and the only way they know how to do so.
To them, a life without kids is unknown to them, and others without kids are seen odd.
since they all have kids, and feel the wonderful joys of being a parent and know ow rewarding it is to be a parent, it is odd to hear one say they are not planning on having any kids, because this is the lifestyle they have grown fond of.
For them to judge you like that is not right, and it seems that they are just too high and mighty with themselves.
Im a parent to a 5 month old, but i do not criticize others who aren't having kids, 1 because im a 19 year old mother and quite a few of my friends range in the same age group, and 2 because i know kids are not for everyone. its stressful being a parent and raising kids, and some people might not be ready to do so. This might be the best decision for yourself and your wife, and im sure you both live amazing fun exciting lives, and might not be able to be doing the things you do now if children enter your lives.
Don't let what they are saying eat you up inside. you know how you and your wife feel, and you know how happy you both are with your choice right now. dont let their comments get to you, because remember they dont know the joys of being a childless couple! they dont have the freedom to get up and go on a romantic holiday with their wife whenever they feel like it, because the thought of who will watch the kids enters their mind. they cant just get up and go out to a romantic one on one dinner with their wife when they please because usually the children tag along behind them. and although to them these things may be a joy and wonderful to do as family, they will never understand how it feels to be a happy childless couple. this is the way they do things, and anyone else not doing this is seen as crazy.
its just the way society has brainwashed.
get married. get a house. have kids.
you might change your mind, you might not.
but dont question your way of doing things because other people are too stubborn to realize there are other ways of living life to the fullest that dot include having children.