@kurtschultz To answer your question, I truly don't know. If they were genuinely curious and kind you probably wouldn't mind. But people are so vocal about their opinions and you might not know until you've already started talking. There's lots of good responses so far that can shut it down real quick! I've got a couple of my own that I like to use.
"I wish we could, I'm not ready to go into details right now" said sadly, they'll probably make their own assumptions and likely drop it.
My favorite is "Oh, are you offering to pay for it?" said teasingly and with laughter, I've actually never had anyone continue to push it after this even though I'm clearly joking.
My second favorite is saying "I guess we'll see what happens!" happily and leaving it at that. They usually assume you're agreeing with them and kinda nod. But ultimately if you're close to them, they'll see down the road that you're serious. And if you're not close, they can assume whatever they want, it won't matter.
My daughter was 2 mo. when my husband got a vasectomy, and if people want to know details about our family planning I gladly give them uncomfortable details. Our siblings know about the snip, but none of our parents do because it's not their business and I just don't want to hear about it. So sometimes they still probe a bit. I think it's actually better now at almost 3 than it was in the first year, so hang in there! I maintain current attitudes by vocalizing what I'm thinking internally. That I wouldn't be enjoying having a toddler so much if I was pregnant, that I can't imagine how difficult a newborn would be with a toddler, that she sleeps so well now, etc.
I think our family members who don't know about the vasectomy think we'll change our minds eventually, but they can think whatever they want as long as I don't have to hear about it. I cared a lot more when she was a baby that I make people understand how serious I am, but it's like anything else people have opinions on and I gave up. I feel much more relaxed not worrying about what they think at all, and steering the conversation in a way I'm comfortable with.