Walnut is a horrible bed wood. It really is not rot resistant at all, at least if left untreated. A walnut tree will rot right through the heart in just a few years from an improperly cut trimming. Powder post beetles also love the stuff.
Trex will work just fine if you add extra crossmembers between the ones that are already there. It even comes in colors.
I'm planning either redwood, ipe, white oak or pine for mine. White oak is grown locally here. I've also seen a pecan bed that looked nice, although it's more of a southern wood. Teak is very good but also very expensive.
