I think a example will be more clearly.As: a boy and a girl fall in love at the first time in their lives,so this love is clean and unselfish. another ,if a boy or a girl have some bad experiences about love, "纯情“ maybe is a difficulty thing, they will think about some other things about their relationship.

字面理解为这个人对感情如一而终，不会变心的，最后只能说这个人很善良

pure