Because you don't look at your family in a sexual way. Similarly in Renaissance, for example, we had some nudity going on in art, but it was done in a tasteful and non-sexual fashion. That's different from outright pornography or erotica, which is what children should not be exposed to.
Like the parent commentor said, it's probably about experiencing nudity from day one. I grew up in a family where you'd never see someone naked, but then, as a teen, was exposed to a family that's comfortable with being naked around each other. I had some readjusting to do, because for me nakedness had become something super private, dirty and sexual. Now that I think about it it makes me kind of angry that I grew up with such a stupid outlook on the human body.
I now do the same to my kids. The key is to do it from day one, so there is never that first memory of “when did you first see your parents naked”. There is no first because it’s been always. There is no shock because it’s perfectly normal.