I'm generally fine being asked when we plan to start our family, provided we're all very much on the same page about how unpredictable these things can be. It drives me up the wall though when people instruct me in what those plans ought to be. The most common ones I hear are "don't have kids! It will ruin your life!" or "wait at least 5 years to have babies," often spoken by absolute strangers.
Is having small kids very very difficult? I'm sure it is! Are they worth it? Most parents seem to think so. I've done other difficult things, and for most of them I've been offered encouragement and support. People mention that these things are difficult, but worth it in the long term. Why not tell would-be parents these things too? After all, while couples have some ability to increase or decrease their chances of becoming pregnant, almost all couples, even those using contraceptives, "risk" becoming parents. Surely it can't be good to start out with years of baggage saying that your child has ruined your life?