Just because somebody is your number #1, doesn't mean you can handle the smell of their #2.
A guy posted on "Am I The A**hole?" about a dispute he had with his girlfriend over the bathroom policies in their home. When they first moved in together, his girlfriend suggested they have separate bathrooms, and he's serious about enforcing it when it comes to her particularly rancid poops.
My (27m) girlfriend (24f) of slightly over two years have recently moved into a townhome together. The place has two levels; the floor level consists of the living room, the kitchen, and a full bath. The top level consists of two bedrooms and another full bath that connects to the master bedroom.
When we moved in my girlfriend had the idea that we should both have our own bathrooms and I agreed. She claimed the upstairs one while the ground floor one was mine. The deal wasn't that neither of us were to step foot in each other's bedrooms or anything; if I get up in the middle of the night and have to pee for instance I just go into the next-door bathroom; I'm not expected to walk downstairs. But we have our own bathrooms in terms of how we decorate them, do our hygiene duties, and fill with our toiletries, etc.
Anyway, we spend most of our waking hours together on the ground floor watching TV or gaming our whatever. When she has to use the bathroom rather than running upstairs she just goes and uses "my" ground floor bath. I don't care if she just has to pee, but, well, she doesn't limit it to that. If she needs to go #2 she does that in my bathroom as well.