In a basic French grammar book one finds:
When reciprocal verbs take a direct object, the past participle agrees with the subject.
Ils se sont rencontrés à Venise.
Ils se sont embrassés.
Ils se sont mariés en mars.
However, by the book's own definition of an indirect object, i.e., one that inter alia follows a preposition, none of the verbs in those sentences are followed by a direct object and hence the part participles should not agree with their subject. 'Venise' is an indirect object since it follows à. In the second sentence the verb is intransitive and in the third sentence 'Mars' follows 'en' and hence is not a direct object. So I'm not understanding why the past participles in the above sentences agree with their subjects.