Good advice for someone on the end of a love triangle looking at a future of likely heart break. Less good for someone in the middle of a love triangle looking to preserve the friendships he has with the girls that like him.
He's not worried if he can deal with rejection. He's wondering about: which girl he should reject and how to do it in a way that minimizes harm to both the girl being rejected and his relationship with them.
It's as true for
their sake as it is for his. As to which girl to pick?
It's fairly obvious. He has no serious preference expressed between the two, Kurumu was more willing to be honest with him, and more importantly,
he already agreed to be in a relationship with her.
That's not a permanent or forever thing, but if he discards it readily or easily in the short run, it speaks poorly of his character. Moka had her chance, had
multiple chances, and played the 'we're friends' card again and again. Kurumu asked, Tsukune gave a response, and that's how things are now. If their relationship fails at some point down the road,
maybe he'll try getting together with Moka later. But letting Moka be the thing that breaks their relationship up would be very unhealthy.
If I was there, IRL, I'd sit all three of them down and have a long talk about how psychology works, very politely, very patiently, and help them understand their own decisions, the motivations for them, and help them decide whether or not they want to let such motivations rule them in future decisions.
But I'm 31, and have a decade and a half of playing 'reliable big brother' for girls with relationship problems, and bro-talk with guys about relationships. I'm not there. Instead we have the single stiffest of all JoJo's (to all the JoJo's I'm familiar with anyways), and his 'standoffish to the point of siege' approach to relationships and social interaction.
Working within the sorts of things JoJo would actually say, is pretty bloody important.