It is sad but also it's really not that uncommon in straight cis relationships for passion to just... dry up.
I know people who haven't slept with their partner in YEARS but it works for them.
However, if one partner has desire and the other doesn't, therin lies the problem.
I would suggest first speaking to your partner, honestly, openly and with the assurance that there is no judgement. Getting a third party (a relationship therapist) involved may help not only to mediate discussions between you both as a couple but also to give your partner the space to work through their concerns or issues alone with someone who can offer practical advice.
Transition is a big deal for both parties, the changes to the body can be quite extreme and sadly not everyone is flexable in their orientation and what they find arousing.
Sex doesn't make or break a relationship, what does is two partners who are not communicating their needs.
I admit, i'd find it very difficult if my partner no longer found me sexually attractive and it IS something that's made me pause and reconsider transition. Realistically though, I accept i'm asking rather a lot of him. But he's trying and ultimately that means the world to me.
In my life, personally, sex matters because I crave the emotional connection. The physical well, you know, I have other methods for that, but the physical closeness is something I NEED.
But everyone is different.
Honestly it could be a blip, they could get used to the idea, or they might not. As with all things, you really do need to talk it through. It's not gonna be easy, but it's rather essential I think.
Good luck.