When Hiroi was acting possessive, I kept thinking "You did this! What are you getting upset for, you idiot?" And that epiphany he had about 'why you shouldn't share someone you like'? I thought "That's what your exes have been telling you the whole time -- you should have figured this out ages ago."
Hiroi aknoledged he had sex alone with Mine sometimes and so he told Take he could do as much(though he lied about the number of times but Take doesn’t care about that really, he didnt even used it once after it was offered). And Hiroi saw the mark and was shocked because he is afraid that his lover change interest from him to Take but he didn’t do anything bad about it, i dont know why you mentionned that one. In no way did he act manipulative and his possessiveness is pretty much the normal level, and considering his past experience it’s very cool to see he doesnt try to stop them.
Also everyone in the threesome got what they wanted. Hiroi kept both his best friend and lover, Mine got Hiroi and Take, for the first time, feels that what had been shared with him is not Hiroi’s, its also his.
This reminds me of 'A Frozen Flower' how the mutual lover suddenly wants his two lovers to come together and bond.
Hiroi to me seemed like he wanted to share with Take because Take is a kid who couldn't think or want things for himself. If he didn't have Hiroi he'd have no friends and would probably enjoy a life with no lovers or relationships. Hiroi probably saw how secluded Take was and wanted the world to love his best friend just as much as he did. Thus,, he liked to share his girlfriends.
His only problem was, he didn't actually think Take would think for himself and eventually develop feelings. He probably didn't originally like Mine he just needed him in that moment and taking him to Take to show his new toy to see if Take would be interested. His mistake was in thinking that Take could never love anyone but him.
In the chapters you can clearly see him waiting for Take's attitude to change, and when he finally finds a lover that Take also wants and could possibly covet to himself, this is when he gets possessive. When people share with others they do it because they enjoy it and they don't expect that person to continuously beg for more. Hiroi happened to share the one thing that Take had finally found an interest in, and realized all along that he didn't like to share with Take, he just liked the fact that if he shared with Take, he wouldn't actually take interest in it, he just liked it because Hiroi told him too.
He doesn't think for himself very well around Hiroi. Hiroi is like a little manipulating prick. Infatuated with the idea that he can tease Take with anything (sex, love, food, sharing everything) and he wouldn't bat an eye nor want it.