I read your argument as: insular religious extremists do bad things! Therefore, we should also deny thinking and responsible adults freedom to live as they plea

I read your argument as: insular religious extremists do bad things! Therefore, we should also deny thinking and responsible adults freedom to live as they please. If anything, the lesson is that making something illegal pushes it underground, which makes it harder to protect those that may suffer from it in addition to punishing those that would gain from it. This is a lesson demonstrated over and over again -- there are even comments about it in the Tao Te Ching.