Are you both paying for the space? If you are paying for at least 1/2 then claim it by putting a piece of tape down the middle of the room (not including bed) and tell them you need this to be respected. If they are not paying for the space, I would let them know that they are your guest. If they cannot keep your half neat they have 2 weeks to find another place, then the lock will be changed. Or, if you are not paying for the space but they are, ask what is the expectation for doing you this favor? Is employment as a maid expected? If not, can a large cardboard box be used for The Pile so others don’t have to look at it?

Certainly no sexual activity is available until a clear agreement is reached. You will enjoy life in the future with your newly acquired spine.

My husband & I have been married 22 years and are still having the Clutter discussion daily. We have calm tones of voice and ask each other things like, “Where can I put this so you can find it? Is this important to you? I’m taking a trip to Goodwill on Tuesday.” We each have our own room, as do our kids. If something is left in the common areas when I am cleaning it is put in their rooms. I clean the inside of the house, he cleans the outside. We both help each other occasionally. We are both messies but leave each other’s stuff alone.

I lived by myself for many years before we met, due to frustration with roommates. It was worth it to me financially/spiritually and in many ways I long for it.
Could you try the Nonviolent Communication method (in caps): “WHEN I SEE this mess I FEEL stressed because I NEED order. WOULD YOU BE WILLING to keep your things on your side of the room?”