This situation kinda happened to me. ok not exactly the same situation, but similar
Before I had bottom surgery, I was dating a guy. He was my first boyfriend, before that I assumed I was lesbian.
I was full time presenting as female, i had being on hormones for about 2 years, so I was quite a way down the road of transition, so I was passing well. Since I am a girly girl, this helped my passing
He was a deeply closeted gay man, so I was the perfect girlfriend for him, to his family and public I was girl so nothing to see here, but the fact I had a penis appealed to the gay side of him. Things started to get complicated when things started to get physical as i did want him to touch me down there as I did not want to get reminded I was born male. The final nail in the relationship, was when I secured my appointment for bottom surgery. I was so excited and he was angry, he accused me of selling out and bowing to societies definition of a woman and I was perfect the way I was.
He knew my plan all along was to get bottom surgery,when I started hormones that was always the plan, I knew I needed a vagina between my legs to feel 100% female.
Well we broke up. That relationship was about 9 months long (we took it slowly, due him being a closeted gay man, and me getting use to the idea of having a boyfriend). Yes that relationship was doomed from the start.
Back to the question, in this Hypothetical but not very realistic, situation.. the relationship will be doomed. Its only so long you can pretend everything is fine. you need to be you in the relationship.
When I met my husband, I was honest from the beginning, that I was born male as I was post op at this time. I did not want to build a relationship on a lie.