When Emmanuel Lebrecht was still a young prince, he fell in love with Gisela Agnes of Rath (b. Klein-Wülknitz near Köthen, 9 October 1669 - d. Nienburg, 12 March 1740), of an old noble family of Anhalt.

After the death of the Dowager Princess and formal assumption of the rule of his principality, Emmanuel Lebrecht sent for Gisela Agnes and they finally married in a secret ceremony in Nienburg on 22 May (30 September according to some sources) of 1692.

The morganatic marriage of a Reformist prince with a Lutheran woman from the lesser nobility brought vehements protest from the Reform Church and the other agnates of the Anhalt principalities.

The agnates refused to accept the marriage as dynastically valid and Emmanuel Lebrecht sued them before the Reichshofrat for libel in 1696; finally, by means of a treaty of 28 June 1698, the Princes of Anhalt-Bernburg, Anhalt-Harzgerode, Anhalt-Zerbst, and Anhalt-Dessau all recognized the issue of the marriage as lawful heirs and all descendants as princes and princesses of Anhalt with all appertaining rights, without however creating any precedent against the prescriptions of the law of the House of Ascania.

On August 3 of that year Emmanuel Lebrecht assigned his wife the castle, city, and bailiwick of Nienburg as dowager for life and made her "tutrix and regent" in case of the minority of his successor, with the full approval of the agnates.