If using the "bridge network" with a bridge or lagg interface, it is not possible for the vm to have network access. Tested on freebsd 12.0 with virtualbox 5.2.32 and 6.0.12
(In reply to sdalu from comment #0) On 12.1-RELEASE, I can choose a lagg interface as "bridged networking" in VirtualBox 5.2.30 and have a partially functional setup: the problem only happens to me when I unplug my ethernet cable or execute 'ifconfig em0 down'. I have already experienced similar issues on 12.0-RELEASE but I can't remember if my setup was partially functional (like in 12.1-RELEASE) or completely broken (like in your case). Could you confirm bridge networking is not working when your MASTER interface is ACTIVE? Also, please have a look at bug 196133.
(In reply to sdalu from comment #0) (In reply to sdalu from comment #0) You could also find the following page useful, albeit quite old: https://wiki.zs64.net/VirtualBox_on_FreeBSD_cheat_sheet
I am sorry for the copy-paste error.
Sorry for my late answer, I'm closing the bug. If I remember correctly it involved a wireless interface which is not supported according to bug 196133 you mentionned