Information
Ensure that the "MAC Address Changes" policy is set 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 VMs from changing their effective MAC address. It will affect applications that require this functionality. An example of an application like this is Microsoft Clustering, which requires systems to effectively share a MAC address. This will also affect how a layer 2 bridge will operate. This will also affect applications that require a specific MAC address for licensing. An exception should be made for the port groups that these applications are connected to. Reject MAC Changes can be set at the vSwitch and/or the Portgroup level. You can override switch level settings at the Portgroup level.
http://pubs.vmware.com/vsphere-65/topic/com.vmware.vsphere.security.doc/GUID-942BD3AA-731B-4A05-8196-66F2B4BF1ACB.html
Solution
From the vSphere web client select the host and click "Configure" -> "Networking" -> "Virtual Switches". For each virtual switch and for each port group within that virtual switch, click edit. Go to "Security" and set the "MAC address changes" to "Reject".