Theory

I think maybe the reason he always says such crude/hurtful and snide words like "I will never love you/sex isnt love/" is basically a form if oppression on his side. His family has always married the mermaids line in Hope's of breaking the curse. I think their jobs is to accompany the mermaid and letting them live a long life with the benefits of having a marriage/a life/ children without the emotion of love behind them. This way, both families can continue to thrive. And this was the way for so many generations. So instead, in order to break the curse-the mermaid and the shamans descendant have to fall in love.

The original story was that the shaman was jealous. Jealous of what? Of the mermaid? No. He was jealous of the man. The shaman was in love with the mermaid. And the mermaid loved herself more than the man. So the man cursed the two people who had killed him, by making it so that the shaman could never have the mermaid, and the mermaid could never find love again. This is still just speculation but I honestly am rooting more for the creepy vice principal than this douchey seme. Hopefully the author tie the loose ends together Σ(っ°Д °;)っ