Yes. The lxml Python module is a critical component of Sigil (even if you have no plans to use the plugin framework). If you're using a version of Sigil from the FreeBSD ports tree, then someone upstream is probably neglecting to include lxml as a dependency.
If you're familiar with installing Python modules, you can either install it (lxml) via your OS's package management system (sorry, I have no idea what naming conventions FreeBSD uses for Python module packages), or use Python's "pip" to install it.
The barest of Python3 necessities for a functional Sigil installation would be the 'lxml' and 'six' modules. But if you plan on using plugins, the full list of Python modules that plugin developers can count on being available on other systems are:
six
lxml
html5lib
regex
Pillow
cssselect
cssutils
chardet
(many plugins also use tkinter in case that's not included with your Python3).
But six and lxml will get you by (though installing the other modules is
highly recommended).
Oops! Kevin beat me!
(though I don't think bs4 is technically needed since our own version of bs4 is included [if the upstream maintainer didn't mess up]).