The children have an age gap of just over a year (both still quite young). Still, sometimes people ask me this question (not as a part of their professional duties, but in general as a part of "getting to know you" chat). I am a single parent, the father (same to both children) is not involved.
I cannot point out exactly why, but this does sound to me as it has an unpleasant undertone. But I am not British, and there were occasions when I misread the situation due to cultural differences.
To me, they effectively are asking whether I have slept with a man other than my ex husband with a tiny baby. Or am I completely misreading the subtext?