I have three separate answers to this question.

Being Transgendered: I am a female, born a female, but one of my very closest friends for about 9 years now, is a m2f TG, what she went through during the transformation was quite a bit to handle, I know that when we first met, she was still in a male body, we dated briefly and she considers herself bisexual and always has, I do know that to relate these two things, she has had more than one partner at one time, but they have been 1 male, 1 female, not two of each gender.

As for any type of relationship in which you are dating more than 1 person, I have been in this type of relationship for a very long time up until more recently, and though it can be stressful at times managing more than one relationship, it can also be fun. It has its pros and cons as with anything else.

Combining the two together, I think that it is personal choice, I see nothing wrong with having more than one partner so long as that is open, and known among the people you are with (I am against cheating) and I feel that it applies for EVERY person, whether they be born in the right body, or born in one that needs to be fixed