@Lisa367 strange.....
When I go to my listing basics (rooms, beds, etc.) there's a drop-down menu choice for shared or private (guest) bathroom.
On the listing it only shows as 'shared' (or in my case 'private bathroom' in the segment 'The Space'. Which is what I would expect to want to know as a guest. You may not like the word 'Shared' as in regards to your bathroom, but you'll have to face it that what you offer is NOT a 'Private guest bathroom'.
It's easy - will the guest be the only living organism sitting on the toilet or using the sink or tub/shower?
yes > private
no > shared
You would be misrepresenting your situation to guests if you marked the bathroom you and your partner are also using as 'private'.
Do know that if you marked your bathroom as 'private', and your guest arrived, realized that they were not the only ones who used it and left requesting refund, then Airbnb would agree with the guest because you had misrepresented the situation.