I think most people ask because media focuses so much on "the operation" that it's what people associate with gender reassignment. The thought of changing one's genitals is also exotic to most people, since they could never imagine doing so (unless they're themselves dysphoric and closeted, whether or not they admit that to themselves). So in many cases, I think people who ask those kinds of questions are simply curious, and not informed enough to realize that it's actually kind of rude to ask about people's genitals.
I generally try to remember what I wrote above and politely respond that 1) I'm preoperative, 2) I'm planning to have SRS and 3) I'm mostly not bothered by questions, but they should be aware that other trans people might be, and that they should consider how they'd feel is someone inquired into the state of their own genitals. If I'm less patient or notice a malicious intent (like the "you're not a real [gender] if you have a [genital layout]" attitude), I either respond with detailed questions about their genitalia ("Do you have hanging labia?" "Do you have narrow foreskin?"), or *very* detailed descriptions of the penile inversion procedure if the person asking is a cis guy, with emphasis on the dissection part. I kind of enjoy seeing the color on their face after a few minutes.
But most of the time, I find the questions to be motivated by genuine, innocent curiosity - and then I usually choose to educate them. If not in the mood, I just kindly explain that yes, I'm preop, but please, consider if you'd ask a cis person the same question. They usually get the point.
