Its odd... bigfoot is a "man" so he should be able to marry etc. I wonder why they wouldn't allow it?
EAxis probably thought that any sexual/romantic interactions between Bigfoot and a human sim (or any other sim-life form, for that matter) would constitute bestiality, on account of his overly-hirsute appearance and the fact that he resembles an animal to an even greater extent than the other life forms. Well, that's my guess anyway.
Then again, if that were the case, then why was the game programmed to allow human sims to interact with werewolves in that manner, even if the werewolves are in their animal state? Yet another mind-boggler for you all to contemplate.