You're not an insensitive or bad person for wanting a puppy over a rescued dog. There are lots of great rescues out there, but rescue dogs come with their own issues, just like a puppies come with theirs. You could tell them you want to make sure you have a pup that is going to be well socialized and tolerate children--sometimes (but not always) rescues don't, or haven't been super up close and personal with kids so you don't really know. You could tell them you want to promote the health and welfare of the breed. Ultimately, no matter which you choose, you're in for a lot of work and a lot of happiness!
People have said that to me before too. My first dog was a rescue--at the time there was no way I could handle a puppy because of time issues. He came with his own set of stuff that we spent a lot of time on, but he was truly a joy. My second was a rescue puppy--she came with a LOT of health issues that I knew about up front. Many people wouldn't have considered her. My third was a puppy from my own breeder. One of my friends was like 'why wouldn't you rescue??!! There are so many great dogs out there." Conversely, when I rescued my first and second dogs, my parents were like "why would you do that, why wouldn't you want a dog that isn't going to have as many issues." They only ever owned Goldens from breeders.
Ultimately, there are people who are just going to be in the "never breed, only rescue" camp. You don't need them to understand, and they might not agree with you no matter what they say. Try not to take it to heart, or let them dictate your feelings too much if you can. In your heart, only you can make that decision for yourself and your family. No matter what, your dog will be a source of joy.
As an aside. People will share this kind of stuff about many dog related things: rescue vs breeder, training styles, crate training, prong collar vs gentle leader vs easy walk harness, dog food, pet insurance, leaving your dog alone or outside. No matter what, many pet owners have passionate beliefs, and often will share them, sometimes unsolicited. A stranger yelled at me on the street for "muzzling' my last dog when she was in a gentle leader. She pulled an insane amount for a 45lb golden. I educated her about them, but she didn't care. It stung, but I knew I was doing the right thing for my girl at the time. You just have to figure out what is best for you and your family :smile2: