Man thinking about Blackbeard from Izzy's perspective is so fucked up actually.
Like on the surface he puts Blackbeard up on this pedestal, he doesn't see Ed, he doesn't want Ed, Ed doesn't appeal to him.
He wants Blackbeard because Blackbeard is success, he is noteriety, no one can touch him.
And for the longest time, Izzy probably believed he was as close to Blackbeard as he believed a person could be. Maybe they never crossed the threshold of a physical relationship with him, but that was fine, because those people never really stayed in Blackbeard's life anyway. Izzy was his only constant.
And then, just like that, Blackbeard meets Stede and a wall just crumbles. He's able to be himself in a way that he wasn't able to since childhood. Maybe ever.
And it enrages Izzy, of course. It makes him jealous and frustrated and scared of losing Blackbeard.
But I think it's also safe to say he is also worried about Blackbeard's mental and emotional wellbeing. He's not being objective about it, but based on what little he knows of Blackbeard's past and how long they've known each other, he probably drew the conclusion that Ed became Blackbeard to keep himself safe (probably true) and that he has to stay Blackbeard to stay safe (remains to be seen) and that letting his guard down, letting others in, and changing as a person means losing everything he built and everything he was (not necessarily true).
Basically, Izzy absolutely has tunnel vision and maybe he's a bit repressed when it comes to his own sexuality, but under his surface level possesiveness and anger, there does seem to be care there. It's just not directed at the true core of who Edward Teach is as a person. He's too focused on believing Blackbeard - and leadership as a whole - has to be something specific and solid and strict and vicious because if you show your weakness, people WILL hurt you.
His fundamental flaw is that he can't respect Ed's right to make his own decisions about what is and isn't worth the risk. He wants Blackbeard to feel safe with him and him alone, but he doesn't realize Blackbeard never will because of all the expectations immediately placed on him.
Stede cut through the armor where Izzy never could by simply allowing Ed to be Ed. It's really easy to see Izzy's good intentions sullied by his own desires, desires he's too afraid to mention let alone pursue. But even if he were able to reconcile all that, he still wouldn't be able to allow Ed to be Ed. The artifice there is of his own making. And he's too blind to see it.