>>281090620
This one still hurts to think about.
Mizore and Ruby can cope, since they love Tsukune, but can also recognise the sincerity of his feelings for Moka enough to respect his wishes. Eventually they'll come to terms with it.
Kurumu, though? Since succubi in-universe literally live on affection, being the third wheel in a love triangle is spiritually and physically crippling Kurumu.