It absolutely IS about their wants, needs, and values. This is still America, and this kind of freedom is foundational to our country. The last thing they need is a bunch of busybodies who think they know what's best for a family they don't know.
It's that they are getting into an area where they are at high risk for having very sick or mentally impaired children.
Do you think they don't know this? Do you think your judgment should be substituted for theirs when it comes to their family? Why don't you make these decisions for your family and let them make the decisions for theirs?
If they have 19 healthy ones, count your blessings but be responsible and don't continue to have children when you are older and it is dangerous.
Dangerous to whom?
I think they are nice people, but wow. Why put future children at risk like that when they already have many children?