Another theory on Laby potty humor: it was just messing with her head.

[modernwizard]

Courtesy of Jen Gagne again: I read your point about how the Labyrinth's 'gradeschool boy humor' seemed like the product of the real life men involved in the production, rather than being in-character for a fantasy of Sarah's. But I think it actually made sense that way considering the whole thing was set up to turn her expectations on end and be less temptingly wonderful than she might've originally expected.