This subject is addressed most thoroughly in “Bad Religion” by Russ Douthat.
Subtitle is “Why we’ve become a nation of heretics”
While it is strange, I think you're missing the reason.
The whole moral relativism and "you can't legislate morality" issue is not about morality (right vs. wrong) as such.
It is entirely and specifically about sexual morality. This one area of life is isolated as one in which there can be no absolute rules. There is never a rationale provided for this one area being uniquely excluded, it is just announced as a fact that it should be.
I think that I have read some of the authors that “Chris” has read, including Gerald Gardner, Alestair Crowley, Raymond Buckland, Scott Cunningham, and Isaac Bonewitz.
If you are going to deal with folks that accepted the Neo-Pagan perspective, it is good to know what they have read, to ‘know where they are coming from.’
On the other hand, the Roman general/historian Josephus would be a good start, as well.
As far as ‘moral relativism’ as a phrase, I find that it stinks. Why? If you had looked a little deeper within the Neo-Pagan thought pattern, they live and breathe the term, ‘personal responsibility’, in all they do in life. There is no “Flip Wilson/Josephine” screaming ‘da devil made me do it’! (The Archetypical Christian denial.) Either they DID it, or they did NOT, good or bad, which is determined by either helping (good), or harming (bad), and I admire them for that.
I think the old addage, “be harmless as doves, but wise as snakes”, applies to a lot of things in life, and you might have ‘not hit the ten ring on this one’.