I can't believe I'm linking to this. I'll get a bunch of porn surfers from Google now.
Edlund and Korn admit that spouses and streetwalkers aren't exactly alike. Wives, in truth, are superior to whores in the economist's sense of being a good whose consumption increases as income rises–like fine wine. This may explain why prostitution is less common in wealthier countries. But the implication remains that wives and whores are–if not exactly like Coke and Pepsi–something akin to champagne and beer. The same sort of thing.
It seems to me that marriage and prostitution both have sociological aspects that aren't easily quantifiable. Maybe that is addressed in the original paper.