Blackmail is not quite the right word. There is no evidence in the story that MM is not free to reject sheriff's offer without consequence (above RH/LJ remaining in jail, which as we've all observed is not necessarily unjust).
We also have no evidence that she is sure there will be no consequences. In her position, if the local sheriff had just arrested your loved one (for valid or invalid reasons) and then called you saying he would free them if you only had sex with him, would you really trust him not to take revenge if you refused?
I agree, but on the other hand sheriff's makes the offer after MM pleads. I think that would suggest that sheriff is seizing an opportunity, not extorting MM or demanding sex.