In our society the question which is thought to define who we are sexually is this one: which sex -- your own, the opposite, or both -- do you like to have sex with? This question is assumed to be central. Beyond that, everything else is seen as secondary.
But even if there is one primary question to be answered to determine the groundwork of our sexual identity, perhaps it is this: what gender do you want to be when having sex? Perhaps what is most important for some people is not which sex they want to do, as it were, but which gender they want to be while doing it.