Praise be to Allah.
There is nothing wrong with any Muslim person reciting Khutbah al-Nikaah before the ‘aqd (contract) is made, but it is a condition that the guardian of the bride be present. He should say to the groom, “I marry my daughter So-and-so to you,” and the groom should say, “I accept.” This should be witnessed by two Muslims, and the bride should give her consent, in order for the contract to be valid.
Khutbah al-Nikaah is the same as Khutbah al-Haajah, which the Prophet (peace and blessings of Allaah be upon him) taught his Companions, and it should be said at the beginning of the ‘aqd. This is sunnah, not waajib (obligatory). The text of Khutbah al-Haajah may be found under Question 2066.
May Allaah bless our Prophet Muhammad.