(Speaking as an SO)... it's also worth noting that transition means *change.* Ultimately you'll settle out at a particular physical stage, and an emotional and mental expression that you're comfortable with, but in the interim, it's a period of flux. And as a woman who's primarily attracted to women, especially, it may seem complicated and difficult (or flat-out impossible) to adapt her sexuality to each new way in which you masculinize, and she may be - consciously or not - unable to keep up.
Which is a long-winded way of saying not quite as interested while everything is changing doesn't necessarily mean she won't adjust beautifully and go right back to jumping you regularly once you're mostly through transition and she has time to get used to the way you look/smell/etc. at that point.
I also wonder if she's very VERY subconsciously internalized that men are the initiator, and so she was totally comfortable routinely ravishing a "girlfriend" but is a bit less so with a boyfriend. (That, too, probably would evolve as time went on with you.) Of course, that would have to be separate from the question of why she's then rejecting your advances.
As far as the "girls grow up" thing, well, it is true that cis girls/women tend to have the highest sex drive during puberty when all the hormones are raging, just like boys do. (The studies about how well and how frequently cis women EXPRESS their sexual desire tend to show that it peaks in the 40s, but that's distinct from pure libido as divorced from sex with other people.) Depending on how old she is, she *might* have been in that phase and is not anymore, but it does seem pretty coincidental.
