Why State shouldn't be involved in marriage #319

I read the article because I was curious of the title "Divorced Wiccans Fight Judge's Order" (the title is from AP but the link is to indystar with different title).

A court order prohibiting a Marion County father and his ex-wife from exposing their son to "non-mainstream religious beliefs"

So if Christianity becomes non-mainstream (as if it wasn't already, at least the "hard-core, extremist" kind), court can order it not be taught after divorce? (Or even during marriage?)