[QUOTE=Wakinyan]
Going back to this original thought of mine, then I have to go back to my original question: Why would he choose Io if he “wanted to include the feminine”. There would be lots of more (in the context) appropriate figures – Persephone/Demeter, Artemis, or even Syrinx if you don’t want a full blown godess, just to mention a couple. That’s what I’ve been thinking about. I can’t fit Io into this, which of course would be due to lack of knowledge from my part (hence a post at the StraightDope).
[/QUOTE]
Right, but you’re forgetting it’s a poem, and a magickal one at that, intended to invoke (and evoke) a specific response. Its intent is fierce and sexual.
“Persephone, Pan!” “Demeter, Pan!” etc. lack that gut-punch of drama and rhythm. Too many syllables and flowiness. “Io” (pronounced “Yo” in this context, though I’ve always heard the girl’s name pronounced “Eye-Oh”) has that gut punch sexual thing going on. Think of your lady (or boy) friend at “that” moment: “Oh! Oh! OH! YO! YO! YO! OH OG!!!”
In “Hymn to Pan”, there’s melodious lines of prettiness “Thrill with lissome lust of the light” punctuated with short, sharp monosyllables: “O, Man! My Man!”. It helps to hear the pounding of chests and drums in a simple rhythm underneath: Bam, bam, bam, bam. Io Pan Io Pan! It’s bringing Pan out of the forest, running, running breathing harder, and harder, pulsing, pressing, pushing, praying in ecstacy to come catch, trap us, hold us in a Panic. The whole thing builds and builds in rhythm until it explodes in a pulsing climax of RRRRAAAAAAWWWRRRRRR** SEX** NOW OG SMASH!!!
Ahem. Sorry, it’s been a while. 
Maybe it’s like Shakespeare in that unless you hear it out loud by a good performer, you won’t “get” it completely. When I read it, I “hear” it in the voice of a good friend of mine who is fantastic at it. When he does it, I want to fuck* him senseless, even though he’s on my “eeeeewwww, like a brother!” list the rest of the time.
*Yes, “fuck”. This ain’t no smooth Barry White “making love” song. Its intent is to provoke raw, primal fucking.