But I also came across a human parasitic worm, Schistosoma mansoni that is also monogamous.
According to Wikipedia, "The adult female worm resides within the adult male worm's gynaecophoric canal, which is a modification of the ventral surface of the male forming a groove."
This method of shacking up seems to work for them, as each female lays approximately 300 eggs a day (one egg every 4.8 minutes), which helps make it one of the most widespread of the human-infecting schistosomes. No talks about needing "space" here -- there's no where to go.
For more stomach-churning parasites, check out 9 Human Parasites You Don't Want Along for the Ride.
Photo via usuhs.mil