May women be witnesses to a marriage?

In the Shafi'i school, marriage witnesses must be two men; women witnesses are not sufficient to validate the contract.
This view is also held by the Maliki and Hanbali schools, and it is the practice in Malaysia. Women may still attend and witness the ceremony, but their presence does not replace the two male witnesses. This is not about lowering women's standing, but a specific ruling in the chapter of marriage according to the majority of jurists. See Marriage contract.