[MunchkinMan]

No, it does not negate the ability to make that choice, since the card does not offer a once and done decision. For an example of how this might happen: someone might play Girl Wrestler with the ability to beat the Monster, only to have it modified beyond the ability to beat it. A player with a female character could decide that they now do not want to fight. If Girl Wrestler clearly indicated that the decision must be made before the fight starts (or some similar verbiage), then the ability to change one's mind wouldn't exist, but I don't feel that playing a Monster when looking for trouble automatically makes such a decision for you.