We can't judge what it's like to live as the descendants of a hate figure, they probably see it as selfish to bring someone else into the world with the same burden

Maybe they see it as their way of punishing Hitler, making sure he hasn't got any descendents left on the Earth.

I say good for them. It's not necessary, but I approve.

Not just Hitler, but descendants of several SS didn't marry/have children for that reason.
One even committed suicide after knowing who his father was, there's a movie about it.