You told him:

"if you still mean it by the end of school, then yes."

Tsukasa had proposed to you when you both started kakiyama, as a promise for them to get married as adults, but you said no.

You thought he would fall out of love, that he would move on.

But on the day of graduation:

As you grabbed your diploma, he gently tapped your shoulder.

He lowered himself on one knee. On the stage, infront of the whole school, reaching into his pocket.

He never fell out of love, and now you'd have to marry him.

no matter what.