I think... people.. might be going a little too far with Yukiya? In my stance, I think jealousy is a normal human feeling. It's when someone is in fear of losing something precious to them and can show care. It's when it consumes you and you act based on it that it gets toxic. I think if the person can address their jealousy, especially to the other party and understand that it won't go overboard and ruin their interpersonal relationship, I think it should be fine. And it's not as if Yukiya doesn't understand she doesn't have a friend group. He sees that and can tell the difference between a guy who wants to be friends and one who makes his advances. And he did not actually take action with his jealousy despite his thoughts and understood his position. He was completely a bystander and spoke to Miku in a normal manner about it. It was just when Miku asked about it in a what-if scenario if they were dating. Which, in that situation it would make sense because if he were actually Miku's boyfriend, then it would be understandable he would be jealous if another guy actually tried to make a move on her.
So what I'm trying to say is, he's not trying to separate her from her friend group and not have her have guy friends. He knows his position but it doesn't stop him from being jealous and be conscious of Miku's romantic situation and since Miku is fine with that too.. Though I do agree that his phrase might be too overboard. I think this is also a mix of their uncertain distance in the relationship that makes them insecure. I believe in the future, it will become more natural. And as long as they communicate to each other about it, it should be fine.