@Milicentbystander72
YANBU. My cousin's daughter (I will call her Lucy,) dated a transgender (girl to boy) some 10 years ago when she was 17.
Just like you, my cousin is open minded, not bigoted, not racist etc, but she was quite worried when Lucy came home and said she is dating Callum (previously Chloe.) My cousin knew Callum when he was Chloe too, (so still sort of saw him as Chloe.)
Callum was 20, and had his own flat. His parents didn't disown him, but were finding his transition hard to come to terms with. So he left home at 18.
It was a tough time for my cousin, and she sadly alienated her daughter for about 6 months, because she said she was uncomfortable with her being with a transgender person. She told her that life would be very difficult for her, and she is struggling to accept the relationship, and her daughter rarely spoke to her after that. (For about 6 months.)
She went to stay with Callum most weekends, and barely made any contact with home. It broke my cousin's heart because she didn't know what to do.
6 months after announcing she was dating Callum, Lucy came home crying her heart out, and said Callum had ended it, as he was going to university in a town 400 miles away, and they would not be able to continue the relationship. Lucy said she wanted to move with him, and would drop out of college and get a job in his town and live with him in halls. But Callum said 'no, sorry Lucy, it's over...'
I don't know exactly why he ended it, whether it was because of the distance, or because she was too young, or because she was quite needy and possessive (which she was at the time,) or whether he just thought the relationship was no longer working. He gave no valid reason, just that he didn't want to be with anyone.
3 months later, he had pics on facebook of him and another transgender person... male to female!
Maybe he just needed to be with someone the same as him. If that makes sense. Someone else who is transgender is more likely to understand him IYSWIM.
Lucy (10 years on) is now 27, and married with a (male born) man that she has been with for 3 years, and has a baby on the way. 
So don't worry OP. It will all be OK. I am 95% convinced your daughter is just going through a phase.