I lack the emotional capacity to raise a child, and my dad really wants a grandbaby, so I get this a lot. I've found that the best way is to make it uncomfortable for them to ask me these questions, such as by making an off-color baby cannibalism joke or asking them an awkward and invasive question about their sex life and why it lacks BDSM. "When are you two going to finally try pegging? I never though I would get a strap-on, but now I have three. Never say never." Of course, this method will make you sound like a sarcastic asshole, so I normally reserve it for when people really won't leave me alone.