It's a toilet! No, it's a doorway! No... according to Julia, who found this listing, this door is the only way into the sun room behind the toilet.
The listing's website is tricky to navigate, so I'll just toss this in here. It's from the same house, and I think it's great. Well, so long as those are the family's kids, it's great. Otherwise it's bizarre and creepy. Either way, though, is it really a selling point?