Remus reached out, unseeing, and grabbed Regulus' wrist. "Don't go," he begged softly.
Taking a deep breath to hold back the emotion he still didn't want to let Regulus see, he closed his eyes briefly for calm before turning around to face his lover. "That isn't what hurts. I'm not hurt because I thought you wanted him. I know you don't. I can sense that. I could feel it when I kissed you.
"I told you, I'm afraid of what I would do, not to you, but to the person who flirted with you. I meant that. It's not rational. I spent the hour you were at dinner throwing knives at a target imagining Severus' face in the centre of it -- and that was knowing he wasn't interested by his body language. I don't know what I would have done if he'd responded in anyway that even hinted at wanting you."
Sighing, he reached out to cup Regulus jaw. "You said before, that day on the grounds preparing for the dragons, you wouldn't try to make me jealous because you knew what it would do to me. But, you don't really understand, do you?" He smiled sadly. "It's not about thinking you would cheat. It's about knowing that, while I couldn't ever truly harm you, I could seriously injure or even kill the person you're flirting with. This jealousy you seem to crave isn't from a part of me that's rational and logical. It's very hard to think straight when all I want to do is prove you're mine.
"Flirting with Severus like that, you opened the possibility that I might cause him serious injury, something I would regret deeply afterwards. It hurts because, in a way, it's the same thing your brother did to me. You were using my condition, my instinctive response, to get what you wanted, and you used Severus to do it, without seeming to care what might happen other than what you wanted." Leaning over, he kissed Regulus softly. "I will do a lot of things for you, almost anything. I will walk out of here right now and tell Harry we can't speak any more if you ask it; I will follow you into a solo war, or to the ends of the earth. I will kill to protect you. But, it will kill me to be in a position where I'm so out of control I harm someone who didn't do anything to deserve it. Especially if I'm put in that position by the one person I trust not to do that to me.
"That's why I'm hurt. Not because I thought you would cheat."