Hello,
The SRM 5.x is compatible with the same Host OS as vCenter 5.x: see Compatibility Matrixes for vCenter Site Recovery Manager 5.1 where this is stated: "SRM Server 5.1.x runs on the same Windows host operating systems as vCenter Server." Once you go to the compatibility matrix for vCenter and Host OS: VMware Compatibility Guide: Guest/Host Search you will see that Windows 2012 R2 is supported with vCenter 5.1 U2 and therefore SRM 5.1 U2.
There is no requirement that vCenter and SRM run on the same Host OS.
Also, by Host OS VMware documentation refers to the OS where our products are run. It can be virtualized, but from SRM server perspective it is hosting it. By Guest OS, we are referring to OS running in a VM where our products interact with it externally - e.g. when SRM performs IP Customization on a VM, the OS inside the VM is a Guest OS.
Best regards
Stefan Tsonev