Sorry in advance for the long post!
I gave some advice last night to a young transgender woman that I am mentoring. Even though her gender dysphoria nearly killed her this year, she was falling in love with a straight girl who could never love a woman in a million years (except as a friend) and wondering if true love was worth not transitioning. I discussed my response with my wife, who agreed that I answered just right. Her first words were "No No No No NO NO NO".
I told my transgender little sister that it is complicated, and everyone is different - some people can't or don't need to transition - but when you are someone who needs to transition in order to live a full and happy life, you are guaranteed to break your own heart and the heart of the person you love if you enter a romantic relationship with someone who can't love the person you really are.
I told her if you are a woman inside, you need to be with a bisexual, pansexual, or lesbian woman, or with a bisexual, pansexual, or straight man. If your partner can't love a woman, even if you tell her going in to the relationship that you are transgender and need to be a woman, she will genuinely believe that the love of a good woman will fix you, that she just has to help you appreciate being a man, and convince you that you are not and can never be a woman, and you two can live happily ever after as man and wife. She will still feel utterly betrayed and bewildered when you transition, and she may even hate you for it.
A person who is not themselves transgender, or has never been close to someone who has transitioned, just can't in their gut comprehend how basic and unchangeable the need to be our true gender is, or how painful it is to put it off or ignore it. So when you change it will feel like a choice to them, even if they are well-read and compassionate people, it will still feel like you are choosing the other woman over them and your life together.
I told her I wish so much that someone could have explained to me the things I understand now about myself, and about gender and sexuality and relationships and marriage and love.
If you marry and don't transition, you could set yourself up for depression, metabolic syndrome, alcoholism, anger, distance, and potential death by suicide or stress. Whether you die slowly or quickly from your pain, or just become the zombie walking dead, you will break her heart and kill or deaden your own.
My wife and I have been together for seven plus amazing, sometimes difficult years, including 15 months since I came out and began the transformation that erased her husband and revealed her wife.
It hurt both our hearts that we were certain we were meant for each other. We have both saved the other's life more than once and in more ways than one. Neither of us would be alive, let alone in any semblance of well, if we hadn't been in each others' lives. In the end, love of course was not enough to reverse the facts of biology, psychology, and reality. We were and are each others' best friends in the world, but even our friendship frayed and nearly broke under the strain of our mutual frustration and grief. The marriage had to end as we knew it, as the man she knew and love ended his life as a man and began a new more genuine, connected life.
Waiting to transition almost killed me, literally, and made me so fragile that a gust of wind could have blown me away. My wife had to save me by letting me know that she would rather have me alive as a woman, than dead as a man. Ironically, the emotional reality has been that our fears did come true in a way-- I really am dead as a man, but I am more alive than I have ever been as a woman and as a friend.
The real reason we are splitting now, but staying close supportive friends, is that she loved the man I was, and the idea of growing old with me, so much, that living with me and being married to me now makes her burst into tears because her heart can't forget what she has lost, even as she is happy that I survived, and genuinely likes me more as a woman and can see that this is who I was meant to be. She resents terribly the decades I survived as a man, and why couldn't I love her enough to survive the rest of my life with HER as a man. Even though she knows I couldn't help it, and would have died without this, she still resents what life has done to US.
My very happiness as a woman reminds her that she believed I was happy with her as a man. It's hard on us both. It's hard on my heart to see that my gender expression - and being happy as the true me - brings sadness to the person I love the most.
It broke my heart to accept that she could never love this woman, me, the way she loved the man she thought I was. It broke her heart to accept that I can't possibly love her enough to stay the man she thought I was, because love can't change that I am not that man.
But after the heartache comes acceptance, and in time, healing.
My only advice to those already married, is be honest, be kind, be compassionate, and don't wait to do what is right for self-care. You can't control the outcome, and no one is to blame. You can love one another, preserve what can be preserved, and let the rest fall away and be mourned, if you are lucky, hand in hand.