splorange, pornography (as opposed to obscenity) is genearally legal in the U.S. What made the MacKinnon Dworkin/"radical feminist" approach to adult content so controversial is it sought to regulate it based on whether it was "good" or "bad" in how it presented women. The kind of relationships discussed on this site, in which females are presented as wanting and needing physical discipline, would be really really bad in this view. The very idea is pernicious, even if the women consent and really mean it, would be desperately unhappy without it.
As the Court ruled in the Hudnut decision, this distinction is not permissible under U.S. law.
I am surprised you say that the only "spanking porn" you ever came across featured men being disciplined. I am aware that such exists, but have not seen it. The amount of porn featuring women being spanked, from spanking romance novels (I was stunned at how much came up when I searched "spanking" on Barnes and Nobles ebook site) to hard edged videos, is huge. There are quite a few people producing spanking art, for example, and most is M/f or F/f.
AS to your last point, from what I can see, there is a fair amount of diversity in CDD. Many people involved in CDD appear to view it as an expression of something fundamental about their relationship to their spouse and God, and not as something that applies to all Christians. I get that in some incarnations CDD is very doctrinaire. The same can be said of secular dd. You can read pages of analysis of how women are naturally submissive to men, etc., on some dd sites. IMO, much of CDD is little different than TTWD, it just has a religious overlay.