To emphasize the point again, the exalted Messenger married only Hazrat Khadija when he was 25, and she was 40. They lived an ideal married life for 25 years. Hazrat Khadija died in Makkah at 65 (619 CE). Three years after her death and soon after migration to Madinah (622), Muhammad (S) married Hazrat Ayesha, a sister of Hazrat Abu Bakr. She was then 48.
During the blessed times, the Central Government run with Shura used to issue a deed of marriage to all women seeking asylum with Rasool (S) and Sahaba Kiraam in order to prevent slander. This is the reason why the Qur’an mentions the exalted Prophet's wives in the plural. Please reflect on the many shades of the meaning of Nikah. Also, consider why NONE BUT Hazrat Khadijah bore any children to the exalted Prophet.