The point is that the
reason why it is important that ISPs are dumb service providers in this case is that they are not just servicing the account holders, and the other people in the household can be severely affected by the decisions of the account holder, but often does not realistically have a voice in the case of "embarrassing" or sensitive topics like this.
The reason for making the argument with respect to the safety of children or spouses here is that the arguments for the filter is the protection of those very same groups. The proponents of this filter have opened the door to this line of argument wide open by making that argument while ignoring that the filter also is likely to harm the groups they claim they want to help. And while there has to my knowledge not been shown that viewing porn causes any serious harm, abuse in various forms is a very real problem. If the people arguing for this filter are serious about the intent of protecting children (which I must admit I don't think they are), then they need to be confronted with the reality that they may be doing harm to the very weakest part of the group they claim to want to protect.
If the filter only affected the person opting in/out of it, I wouldn't see it as a big deal - in that case it would not remove freedoms from anyone.
But the most compelling immediate problem with this filter for me is exactly that it effectively takes the choice of having unfiltered internet away from a large group of people that are often vulnerable. That it has been shown to be overly broad in a away that takes away access to particularly sensitive sites makes it substantially worse.