Depending on the networking settings used, there are actually *two* distinct computers connected to the port, the Mac and the virtual computer. Using Bridged networking they are really exposed as two distinct computers, two MAC addresses. In a way, yes the Mac OS X host acts as a hub or switch (L2) between itself and the virtual machines it hosts and all of these are connected through a single shared cable to the switch port.
Shared networking is different and might as well be your issue (especially since it is the default mode if memory serves when you install a Windows VM at least). With Shared mode, the VMs are in a distinct isolated segment behind the host IP pretty much like home computers are isolated behind a home appliance modem/router, with NAT to access the outside world.
So... If you're set for Shared mode networking, I'd first set it for Bridged mode networking. Chances are it might be more compatible with your switch policies.