I get asked SO often if twins run in my family (usually they look at both babies, ask if they're twins, then ask if twins run in my family) that I have to assume that a percentage of the questions have that "ulterior motive" aspect and a percentage are just knee-jerk curiosity kind of default questions. I think some people must have that question as a way of sussing out something (such as the people who say, "So you didn't have fertility treatments, then?" if you confirm that twins run in your family), but I don't think it's true of all or even most.
But "most" (greater than 50%?) doesn't mean much. It is such a common question (for me, at least) that fifty percent either way would be a significant number....twenty percent would seem significant, too. Even if only five or ten percent of people who ask me that question are "asking" something else besides just if we have a family history of twins, it still would be quite a few people.
But I agree that it is one of "the" questions to ask when you see multiples. And I do think people tend to make default assumptions partially based on their experience....including what gender they assume or expect, when asking about my twins. If they know someone in X situation, or if they've read some articles about particular incidence of something (rise in twinning due to fertility treatments), that scenario pops into their brain when they see multiples.
Moms of twins are especially likely to talk to me when they see me with my babies, and they are quick to tell me that they have/had twins, too. Then they tell me about their twins (who now are 9, 16, or 25, or 36), and I've noticed they often will ask "Boy and a girl?" if they had b/g twins themselves, etc. Then again, my guys have been wearing their amber teething necklaces more now that they have stopped grabbing each other's, so maybe the sight of the necklace makes people assume girl....
I don't know what the "searching" or "fishing" people think of my response to their "run in your family?" questions, because I generally react with a little surprise and sort of reflect and say, "Yeah, I guess so, we actually do have twins on both sides. I had some on both my mother's and father's side, and J's sister had twins," but I say it as if it's all anecdotal and somehow unrelated. Occasionally I elaborate and say "But it's supposed to be fraternal twins that have a clear genetic connection, or an inherited tendency to have double ovulations. The reasons for identical twinning aren't clear" (I used to do this more in the beginning, until I realized HOW OFTEN this question would be asked and just started saying, "Yeah....I guess so" in a "how does this relate to anything" kind of way. And I am realizing that that slightly awkward-seeming reaction probably makes the people who are "fishing" with that question assume that we used assisted reproduction.
But it always feels like a question from out of left field, like I can answer it but exactly how does it relate and what am I implying? And I always stumble a bit, as if I've been asked "Do you walk to work or carry your lunch?"