That's a baseless assertion -- legality and morality are independent and only coincidentally aligned. Unless you want to start telling us about the ethical necessity upheld by the farm subsidy bill.
Clearly it would be immoral for you to intentionally provide a source of liquidity for stolen goods. How does that show anything about your gambler?
They are both separate examples of contracts that are consensual and without coercion, but are immoral, both for different reasons. My goal was to find a counter-example to the statement that "All consensual and non-coercive transactions are ethical.". Once that example was found, expanding from there.
Clearly it would be immoral for you to intentionally provide a source of liquidity for stolen goods. How does that show anything about your gambler?