<<However, the person who is at the receiving end of the explanation or proposition is always an indirect object. "She proposed marriage to him.">>
But "propose" can't take an indirect object in the sense of, "She proposed him marriage." The distinction here is that nouns and pronouns in English can often take what one might call the "dative case" and serve as indirect objects without needing a preposition, as in "He gave *the dog* some food" and "She told *him* a joke". But nouns and pronouns can't take this dative form with "propose" and "explain" - in order to serve as indirect objects, they have to be preceded by "to".
But "propose" can't take an indirect object in the sense of, "She proposed him marriage." The distinction here is that nouns and pronouns in English can often take what one might call the "dative case" and serve as indirect objects without needing a preposition, as in "He gave *the dog* some food" and "She told *him* a joke". But nouns and pronouns can't take this dative form with "propose" and "explain" - in order to serve as indirect objects, they have to be preceded by "to".