Not all questions are harmless.
If we're being honest, we all make assumptions about other people, right?
We look at their skin, their clothes, and their car, and we make guesses about them that we don't even realize we're making. Everyone does it.
You ask a pregnant female coworker if she'll keep working after the baby is born — but you wouldn't think to ask that question of a guy who was about to become a dad.