I'm sorry you're going through this, OP. I think there are a heap of things to separate out here.
First, i think you can be supportive of someone without necessarily being happy. If he'd come out as gay, for example, I don't think anyone would expect you to be doing flips with excitement at the end of your relationship. But you can still be supportive of someone trying to come to terms with their identity, though it is incredibly challenging. And I agree that there needs to be more support for partners.
Second, being supportive doesn't necessitate taking part in sexual activity that you do not enjoy or are not turned on by - and, of particular importance in your account - does nothing for you at all. My partner likes to play with his gender presentation, and we play around with who is dominant and who is submissive. But we both enjoy that. That's the key difference. You've been supportive, you've tried it out, you haven't enjoyed it. And that is really absolutely fine. It doesn't make you 'boringly heteronormative'. It makes you heteronormative in your sexual preferences and gender attraction, and that really is ok.
This is a hard thing that you are both going through, and it does not making you anything-phobic to feel sad at the change in your relationship. I suppose it's probably fairly unsurprising that someone going through a huge journey of self-discovery becomes pretty self-absorbed, but you don't have to tolerate that. You can allow the exploration, but remind him that his individual journey is affecting your joint experience as a couple.
These stories always remind me a little of a couple i knew, where the woman had a catastrophic brain injury that entirely changed her personality. her husband struggled for years to deal both with his grief of effectively losing his wife, and his guilt of feeling like he was doing something wrong in not loving her anymore. They had a lot of therapy together, eventually split but very amicably and are now very close friends. it is possible, but very hard.