The only other way I can read it is that Mitsuru isn’t gay, but actually has no interest in sex or relationships
That's exactly the way to read it, I reckon. An important thing to remember here is that, among the Parasites, Mitsuru and Zorome are the most indoctrinated, each in their own different way.
Franxx is very obviously a show about sex and relationships
I totally agree with that point, but with the proviso that the two elements I've underlined in your quote are sometimes held in separate hands and studied separately.
I dislike him so much because his reaction to the Hiro thing is pure melodrama
It is undeniably melodramatic, I think, yeah. Even as a fan of the show, I cannot deny that. I'll play devil's advocate for a bit here, though, and try to see events from Mitsuru's perspective.
I suppose that, first off, Mitsuru had more of a struggle than anyone else when it came to living up to his reason for being. The Parasites were created by Papa to pilot FranXX. That's what Mitsuru knew, but his own inbuilt aptitude for syncing with a FranXX was low — so low, in fact, that he elected to undergo an very risky injection to increase his yellow blood cell count. The success rate is extremely poor, and the outcome is potentially fatal.
Things must've looked pretty bleak for him. Other candidates who struggled with their low aptitudes mysteriously disappeared from their home of the Garden, spirited away without warning or ceremony, never to be heard from again. Those are high stakes and must've made for a very stressful situation to have to live through, so the young Mitsuru sought to give himself something to look forward to should he be lucky enough to pull through the procedure — a light at the end of the tunnel. That light was a pact made with Hiro. Mitsuru was inspired by Hiro and looked up to him. Maybe... Maybe he could get to pilot a FranXX with him one day! What an amazing thing that would be; he would be able to fulfill his purpose and not have to disappear somewhere probably not good.
And wow, would you believe it! The procedure was a success! Mitsuru had beaten the odds and all the doubt and uncertainty had been swept away all at once. Excited by his new-found future, he ran towards that bright, shining light.
Only to see it immediately snuffed out and be plunged back into darkness. This is why Mitsuru has shunned emotional closeness with his peers ever since, and is what has made him so cold and aloof. He never wants to have to live through another situation like that ever again, where one individual can so drastically and all too easily alter his own path through life.