Freud was a great advocate of the bisexuality of all persons. It was no surprise to him when he studied dreams in which romantic content existed that was contrary to the dreamer's usual sexual preference. Many times, this romance is a form of "mirroring."
Mirroring is the desire to find affirmation of a particular aspect of yourself in a particular sort of relationship. The romance contrary to your usual preference should be set aside initially in order to discern what sort of affirmation you may be seeking from this liaison.
Another possible reason for a dream that involves bisexuality for a typically heterosexual person is that the anima/animus is flexing itself.