Is my Mac partition secure if it has FileVault 2 but the Boot Camp partition is unencrypted?
Yes -

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.