There's probably no cure to how Emet-Selch gets possessive. It's just something he has always been, longing to covet Hythlodaeus completely to himself. At one point he probably also felt similarly to Azem. In fact it's his turn to be a little clingy after the kiss, burying his face into the crook of Hythlodaeus's neck protectively.

"I know that, it's the principle of the thing really." He complains into Hythlodaeus' shoulder, hugging him close. "I know your heart is mine but the thought of sharing you with the unworthy..." Unbearable. Horrific.