For a long time I mostly used the first answer. I only used the latter answer whenever I felt defensive. When I started feeling more at peace with our decision to live without kids, I was mad at myself whenever I automatically blurted out "not yet." Now when I ponder upon it, it was actually "easier" to say "not yet" because more often than not, upon hearing the answer, others would simply respond, "Oh, you still have time" or something like that. And the topic ended there.
However, whenever I offered the latter answer, more often than not, people would offer unhelpful-but-well-meaning suggestions. One guy I hardly knew even nervously joked about our infertility because he didn't know what to say (I had a hard time forgiving him and for a long time I kept on repeating the joke over and over in my head which made me fume even more, but now that I'm typing this, I realize that I have no more grudge against him. HALLELUJAH!!!! I'm FREE from that shackle!!! :-D)
Anyway, I realize that these days my answer has changed into a "no". I was disappointed at myself for a while for having automatically given the answer "not yet" on a few occasions (not many people here ask that question unlike what happens in Indo), so I tried practicing the answer "no" a dozen times in my head, but I never had a chance to use it yet.
However, last week my much younger coworker (21 y.o.) asked that question and I said no (and I didn't feel the need to explain anything to her = I wasn't feeling defensive).
She asked, "Why not?" I explained to her that it didn't work for us.
Then she said, "You can always adopt."
I explained to (educated) her briefly that adoption was a complex process and that our ages also had an influence on it. And I added that we were fine even without kids.
She then asked me how long we'd been married and I replied almost 7 years. Then she stopped asking me more questions because we had to get on with our work. :-)
P.S. Here's something I made when I was raking the autumn leaves outside. Click to view it in a bigger size.