If you believed with 99% certainty that you were a Boltzman Brain, what should you do? The smart thing would probably be: Whatever you would do if you weren't a Boltzman Brain. You can dismiss the hypotheses where you have no control over the future; because it's only the ones where you have control that matter on a decision-theoretic basis.

The Boltzman brain has a fraction of a second to a few seconds to exist. if there's a sufficiently high chance you are a Boltzman brain it may make sense to maximize your very short term utility. One obvious way to do so is to spend that time fantasizing about whatever gender or genders you prefer.