You are not alone, I find him kind of annoying too, he is clingy, selfish and rather immature. A big foil for Kamio, who is the same age and is not those things.
Yea compared to Jun hazuki is like an adult almost mentally anyway. In the beginning he hateD Kou now he's very into him lol plus Jun does not know that Hazuki makes out with his brother and bites him all over the place when they have threesomes (⌒▽⌒). Wonder what he would do if he found out about it? He's already pissed of at Touya for taking his brother but then Hazuki 2 he just might kill someone
Kamio actually seems less grownup than Jun. At least Jun never tried to force himself physically on Kou. Plus he is way more considerate of Kou's feelings than Hazuki has ever been. Having said that the way Jun clings to him can be immature in ways that Hazuki isn't but I think that just makes his character more fleshed out. By no means do I think it makes him annoying.
Am I the only one who is kinda annoyed by Jun??? I like him, but i find him annoying at times too...