My girlfriend jacks me off with her down comforter

it's not a down, it's a duvet. get it right. ;)

You sir, are a lucky motherfucker!

Sounds normal. Lots of guys like different textures, you're being 'encased' and rubbed with it, why wouldn't it feel nice?

What's a down comforter?! :/

I have no idea what this is.

And down is feathers. Particularly the soft feathers that are underneath the rough exterior ones.

A duvet is the cover. Down is the filling.