In the final scenario the equities and expected outcome can vary wildly.
But, that fact and the overall odds of those variances occurring is known when the decision is made and should be factored in to that decision.
In the end, it's your club and your rules. But, I disagree with the rationale. If the players want to bet whether the turn will be a black card, I'm not going to prevent them from making the bet pre-flop only because the flop would certainly affect the odds.