The vCenter Server must set the distributed port group Media Access Control (MAC) Address Change policy to "Reject".
If the virtual machine operating system changes the MAC address, it can send frames with an impersonated source MAC address at any time. This allows it to stage malicious attacks on the devices in a network by impersonating a network adaptor authorized by the receiving network.
This will prevent virtual machines from changing their effective MAC address and will affect applications that require this functionality. This will also affect how a layer 2 bridge will operate and will affect applications that require a specific MAC address for licensing.