Hello Hectic.
Welcome to Susan's, this a great place to be and I hope that you enjoy your stay here and that we can be of help.
I do not intend to trivialize your issue, but your's is not a unique problem, I know that this is not uncommon with mtf transsexuals. Many of us are not sure of our sexuality in early transition and often late into it. It is something we obviously have to thiink and wonder about. Personally I did not have any desires or feel any desires towards men, and I am still very much attracted to women, so I consider myself a lesbian. For me I didn't feel the need to experiment. My problem is similar to yours, in that my wife has flatly stated in no uncertain terms that she is not a lesbian so that means intimacy and sexuality between us doesn't exsist. We haven't let this come between us in any way, and we have no plans on breaking up, but we have discussed the possibility that one of us will find someone who would satisfy our sexual needs.
I think that you and your spouse should really discuss this issue before you agree to any form of separation. You need to know what her intentions will be if she does find that she is attracted to men and how it will affect you and your relationship with her. Would you want to stay in the relationship or not. Personally I don't think that she needs to experiment to find out if she is attracted to men as she has already experienced this. I believe that it is something you are born with, and not something you've developed with being transsexual. It really sounds as though she is bi-sexual, and wants the freedom or permission to indulge this.
The other way of looking at it is, what would you do if you were a married couple (non trans), a man and a woman, and your husband asked you if it would be ok if he went and experimented to see if he were attracted to men?
It would be cheating, I really don't think that she can have it both ways unless you are into swinging. How would you feel, you are just as important to the relationship as she is, make sure you are not being manipulated in to agreeing to something you are not comfortable with.
You definitely need to discuss this before any decisions are made. You didn't mention if she was in therapy, but if not, maybe you could suggest that she does seek it where this can be discussed with professional guidance, you could also be included.
I am no expert it this but I hope that I may have been some help. I'm sure that you will get other advice here at Susan's. Take it all, and go from there.
Take care,
Steph