Kyoutanti is accustomed to people touching him roughly (bad homelife, bullying, etc.) and so even though he's happy when he and Iwaizumi get together, he's still expecting more of the same (after all Iwaizumi's been rough with him before and he's seen how he and Oikawa are). But it takes Iwaizumi about .02 seconds to recognize he doesn't want to be one of those people to Kyoutanti. Cue lots of fluff and gentle loving sex and Kyoutanti just falling apart and melting under Iwaizumi's hands.