Read all of the answers, and most/all make sense.
For me, I wear mostly by choice, but also by occasional need. My driving job has difficult moments, that I handle however I choose, depending on the situation. That would be the "choice" option. But, I honestly do have IBS issues, and although those events are rare, I wear protective clothes all the time just in case. I've had a couple incidents that we're mitigated greatly by being able to toss a pullup away. Hate those!
In general, I can't imagine anyone ever asking, and I do wear in stealth mode, but if someone did ask, family or otherwise, I'd tend towards the need option/explanation, with as few details as were required to satisfy their curiosity. If they really delved into it, I might figure they had an interest, for their own reasons, and I'd try to explain the usefulness, protection, and comfort they provide, in an uncertain world.
If anyone came at me from a negative aspect, I'd remind them that there are laws protecting the disabled (ADA, Americans with a Disability Act), and that they were infringing on my human rights to self preservation & emotional protection. My IBS is well documented, and, again, in my job, like an astronaut, or a deep sea diver, there are times when nature calls, and my ability to control that is limited.
I'd add that I've worn something, usually pull-ups & protective pants, daily, 24/7/365, for over four years, in VERY CLOSE contact with other humans, and nobody has ever said a word, or acted in a way that let on that they knew (my wife knows, obviously). It just hasn't happened! It's private. I keep it that way. For people that wear by choice, it makes sense to just keep it that way.
If you're keeping it from family, and don't have any implied deniability, then you need to be very cautious/stealthy, or very open with your specific choice/emotional needs, and/or suffer the consequences of letting them find out.