Hi Crazy-for-the-person
You're in a situation with many challenges, and there are probably no simple easy answers about to jump out at you. Please permit me to recast your final question, and offer you a possible alternative view of your situation.
It is sometimes hard, even for the person themself, to clearly distinguish and be able to identify the reasons behind wanting to dress or be more feminine. Often it's a mix of things, including fetish, identity, perhaps the liberation of being "someone else" for a while - who can tell?
I truly truly understand your feelings of insecurity and frustration. Even if you do establish limits, when might these be tested, and how would you react? Again, it's hard to tell, I guess.
Perhaps the most positive thing is to deal with all of the spoken and unspoken issues. Nobody can advocate a course of action except the two of you, and I guess you are feeling that if you open the lid fully, you'll have no idea what kind of genie might spring out. Yes, if the person you fell in love with was a man, there is a big question whether you would have the same intimacy on any number of levels if he was something else. Although I am (now) a hetero woman, I truly believe that intimacy between two women is far stronger than that of other gender combinations. Just my feeling...
If you think you can manage this, may I suggest that you do open the lid fully, knowing that it might take you on a weird and wonderful journey. So many of us live our lives scared of the consequences of possible actions, or of what society might say, or of what we truly might think or do if we liberated our minds. I'm not saying that reckless abandon is the way forward, but you might have the journey of your life, full of the richness of uncommon experiences. Perhaps start by seeing the right type of therapist, together, and gradually explore what you both have and want from your relationship and your lives.
Julia