Your problem with environmental degradation of the materials can be very easily solved without poly or pressure treating by adding a roof to your deck so it's covered. In addition, it's fine if your standard wood begins to degrade in the environment because it's just a matter of replacing a decaying plank with a new one.

I don't know how long you were looking for your deck to last without maintainence, but I'll be honest, I think you're really underestimating the life cycle of untreated wood (when sanded & varnished as needed).