The following was excerpted from a post to SGM, 3 Jan 1999, by Leo van de Pas:
Philippe I, King of France
At age twenty, Philippe I, King of France, married Bertha of Holland, the step-daughter of his first cousin. This marriage had been arranged to seal the reconciliation between the king and his first cousin, the Count of Flanders. However, it took Bertha about six years before she produced a daughter, Constance, which was not the hoped-for heir. Arnoul, a holy hermit of Saint-Medard in Soissons who was always consulted on family problems, prayed to heaven; however, it still took another three years before the heir, the future King Louis VI, was born. He was followed by three more sons.
Twenty years after the marriage, Philippe imprisoned Bertha in comfort in the chateau at Montreuil-sur-Mer. He then wed the still-married Bertrade de Montfort l'Amauri, wife of the Count of Anjou, and they produced four children. It is lost in time whether she seduced him or he her, but most likely Philippe had an understanding with the Count of Anjou. In any case Bertrade was more than willing as she did not want to be "sent away like a whore," as her husband had done to her predecessors.
Philippe's remarriage caused a sensation but not disapproval. The only one who caused problems was Yves, bishop of Chartres, who had been appointed by Pope Urban II without consultations with Philippe and this had been resented by the latter.
The King had invited all bishops to his second wedding but Yves declined, referring to Philippe as committing bigamy. Although Philippe had married with the blessing of the Archbishop of Reims as well as the Papal legate, Yves wrote to the Pope who then forbade the bishops to crown Bertrade and told Philippe to cease all relations with her or else be excommunicated. Next, Bertha, his first wife, died and Philippe gathered two archbishops and eight bishops in Reims who all confirmed the royal second marriage. The Pope also put pressure on the womanising Count of Anjou who then obediently complained about the king's committing adultery with his wife. In 1096 Philippe pretended to have broken with Bertrade and consequently the excommunication was lifted.
However, when it became obvious in 1099 that Bertrade was still with him, the excommunication was renewed. It took until 1105 before peace was restored and from then on Philippe and Bertrade remained together till Philippe died in 1108.