The marriage of Imam Ali (AS) with Lady Fatima (AS) was ordained by Allah (SWT) when He ordered the Prophet Muhammad (SAWA): ZAWWIJ ALNOOR MINAL NOOR ( Arrangvthe marriage of the Noor (Ali) with the Noor (Fatima).
This Hadeeth was narrated in many Sunni books like Al-Mustadrak ALAL-Saheehain by Al-Haakim Al-Nisabouri (Volume 3, page 125).
This means that that marriage marriage was by order from Allah (SWT).
Wassalam.