Sealing a Stained Poplar Bed Frame

Is there a way to maintain this kind of raw wood look and still seal the wood from damage?

Yes, think of cutting boards, those are raw wood, and they can be maintained, while being exposed to moisture and a other materials. Cutting boards need to be oiled to help keep the water from washing out the woods natural oils, and to mitigate over-drying and contracting wood which will split.

In this case, it's not a cutting board - so I recommend a wax based finish. They are nice, safer to handle, and the usually smell wonderfully of citrus. A wax finish will look more like raw wood than almost any other option.

Like a cutting board, you would be wise to re-apply, but you could do this every year to every 3 years, depending on your climate.

And why is that necessary?

Wood has a relationship to water. Think about where wood comes from, trees. Trees are in the business of moving liquids up and down, out of the ground, and back down. A tree's hardwood forms out of old softwood, so much of the structure of the wood is the result of a tree's need to move moisture.

That is to say that wood will interact with its environment, the air it's in, and the moisture content (or lack of moisture) in the air. It will expand and contract, which puts strain on joints, and fasteners, and will cause them to loosen over time.

Sealing the wood with a finish does not stop this interaction, but it limits it, and is a good way to help the furniture we love to endure for many years.

I usually add a finish to seal the wood but it would probably be fine with out any other finish sense it will not likely be subject to hard use or abuse. But I would use something like a wipe on poly as they provide a very thin and fairly durable finish, if the satin finish is to glossy it can be rubbed with an fine abrasive pad.