The truth? Well we don't know. We know we're not ready now, but we don't know if that'll change in a month, a year, or five years. Maybe we'll never be ready.
Not everyone has kids, and that's a good thing. It makes up for the people who pop out twenty children. The world is overpopulated as it is, we could use fewer babies. It is wrong for our society to think just because you're married you should have children. There are plenty of horrible parents out there that should never have had kids.
Also, it really is none of your business. Why do strangers care if you have children or not? It doesn't affect them. They're not going to be grandparents. They'll probably never even see the child.
Now when family asks, it's still rude, but at least it's because they want to be an aunt or uncle. They might actually see the baby. So I can understand why they might ask, maybe once, but to repeatedly ask? Well maybe they forgot your reason and want to ask again, but at some point it's not longer justifiable and it becomes just plain annoying and rude.
I also think when people ask this they are looking for an argument or some way to pressure us into having kids. They'll say things like "oh you're never ready for a child, just do it." Yes, that's a great reason to have a child, if you want to be an irresponsible adult. Just because one can, doesn't mean one should. Sometimes I just want to slap people.
A lot of time people say you'll love having kids because they love their kids. FALSE! Not everyone is the same. Not everyone is meant to be a parent. I personally don't like kids. They are annoying, loud, germ spreaders. To be fair, it might just be the parents I don't like since it's probably their fault for not keeping their children under control, but that's just proof that not everyone should be a parent.
When and whether or not we have kids is our decision. It is no one else's business. So if you're one of those people who feel compelled to ask every couple you know, "When are you having kids?" Take a moment and think, why do I care? What kind of response am I hoping for? And if you do ask, accept the couple's answer and move on.