'No'. Usually while laughing.
If they pushed I explained that I barely coped vomiting 20 times a day for the whole 9 months the first time, and couldn't possibly imagine doing it again with a child to look after at the same time. Plus the birth was not an experience I wanted to repeat.
As for being an only child, we're very lucky she has a cousin nearby close in age, who is also an only, so best of both worlds, like a sibling you can return at the end of the day.
People seemed to stop one she was about 5, but I'm in my late 30s so that's probably a factor as well.
I imagine it's so much worse if you're trying / having fertility issues. I conceived writhing a year of trying, with no complications, but by the time I did I was sick of fielding the 'are you having children' questions.
I used to say 'my parents tried for 16 years before they had me, we've only been married two, I'll let you know if it happens, I promise'.