They need to accept that there is an anomaly in your brain; a pull that draws you towards the opposite gender in two separate and distinct ways.
I recall since the time of puberty looking at women and being drawn to them physically but also desiring to emulate and dress like them. I would admire their feminine beauty all the while wanting to possess it for myself.
I understood this to be a contradiction so I strived to suppress my desire to emulate them as a much as I could.
Today I no longer see a contradiction and reconcile my feelings by separating them into distinct parts; one sexual and one identity based. I suppose you could think of me as a male lesbian in that regard and I can now understand why many MtF transsexuals continue to be attracted to women even post transition.
It really is a truism that gender and sexuality are not necessarily linked and you need look no further than gays and lesbians for confirmation.
That final bridging is what allowed me to find peace with this contradiction which resided within my brain. I could not make sense of it for the longest time but eventually it clicked into place.
Once I accepted the identity aspect and truly owned it, I found I could stop weighing the idea of transition. Even as I reside in a male vessel I could still accept that there were two sides to me which can share the same physical space.
But even if the dysphoric comes to some sort of resolution, the partner needs to make a significant effort to intellectualise what has happened because she desires and is attracted to your maleness. There is no easy way to accept the contradiction that is before them.
I am very admiring of women who can see past the physical and look at the whole person but I also realize that there are biological trappings in the way of accomplishing that feat. After all, we are animals with instincts and those are not easily discarded or ignored.
So if you are in a successful relationship despite this challenge, you are very fortunate indeed.