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.