With a Macbook Pro Retina running OS X 10.10 and Boot Camp Windows 7, my Mac HD is encrypted with FileVault 2 and the Boot Camp partition is unencrypted. Does this weaken the security of the Mac HD partition?
if you ask if Mac HD was safe. The Bootcamp partition, however, is not. Any attacker could easily gain access to your Windows system or any other unencrypted partition on your drive. Make sure not to store sensitive information there (e.g. WiFi password, etc.). But normally the bootcamp partition does not contain information compromising the security of your filevault2 encrypted partition (Mac HD in your case). Those two are independent.
So in short: Having further unencrypted partitions on a filevault2 system disk does not degrade your security.
It is a possible back door since not complete disk is encrypted.
The second issue with FileVault 2 is that it only encrypts the startup drive. If you have additional drives or partitions, including a Windows partition created with Boot Camp, they will remain unencrypted. For these reasons, FileVault 2 may not meet the stringent security requirements of some organizations. It does, however, fully encrypt the Mac's startup partition, which is where most of us (and most applications) store important data and documents.
I will not go in to how to, but rather point out following:
In case of full disk encryption you can not proceed at all without providing the key.
In case of partial, you are allowed to proceed, to the windows partition and have access to computer operations.