It took me a while to get started writing today, but I did finally get to it. About 700 words so far, which, with the 2000 I did yesterday, brings the grand total up to around 63,000. I finished Part III yesterday, and started on Part IV today. I'm not sure if things are going a bit too slowly; I might be able to collapse Parts III and IV into one section in the next draft. But I'm not going to worry about that now. Right now, the rule is to just keep going. I admit, last week, I did go back and drop a new 500-word section into one of the earlier segments, but aside from that brief lapse, I've been pretty good about just chugging along.
I'm having a bit of a hard time figuring out how much explanation to put in. Another way to put it would be that it's not so easy, accommodating multiple audiences. I'm trying to simultaneously keep in mind two radically diverse audiences -- people who have been entirely within a monogamous paradigm and have essentially never heard of poly as an option, and people who have been familiar with and perhaps practicing poly for a decade or more. I want to put in enough complexity and subtlety to satisfy the latter group, without totally losing the former group. I suspect the answer is going to involve showing this to lots of readers. :-)
Anyway, I'm just avoiding the problem now, so head down and back to work.