It is a widespread practice to deliver a sermon known as a Khutbah during a wedding. However, there is no actual requirement to deliver such a sermon at a wedding.
Prophet Muhammad (PBUH) never attended any wedding specifically to deliver a sermon, nor was he ever invited to do so.
There is no evidence that he delivered a sermon even at his own daughter's wedding.
Nevertheless, the Prophet (PBUH) did deliver sermons in gatherings of people whenever he deemed it necessary. Based on this general permission, if it is believed that people would benefit from a wedding sermon, there is no objection to it. However, the validity of the marriage is not compromised if no such sermon is delivered.
Witnesses
There are several Hadiths suggesting the need for witnesses during a marriage; however, all of them are considered weak. Yet, the Quran does instruct that witnesses be appointed during transactions.
The Quran (Verse 2:282) states that two witnesses should be appointed when lending money.
Allah states in the Quran (Verse 4:6) that witnesses should be appointed when handing over the assets of orphans to them.
God also instructs that witnesses be appointed when making a will (Quran 5:106).
Based on this, it can be understood that witnesses are essential for a marriage as well.
Just as witnesses are required for other transactions, at least two witnesses must be present at a wedding. This is to prevent a situation where one party might later deny the marriage, and to ensure that those engaging in illicit sexual relations cannot escape accountability by falsely claiming to be husband and wife when caught.
Witnesses are needed to testify to the facts exactly as they occurred should a dispute arise later; they are not appointed merely for the sake of ritual. To state the matter exactly as it is, all key marriage negotiations and the exchange of commitments must take place in his presence; he is the one qualified to serve as a witness.
He must be aware that the bride’s consent has been obtained, that the Mahr (dower) has been given, and what the specific amount of the Mahr is. It is meaningless to designate two individuals present at the wedding as witnesses if they are unaware of these details.