(This is an old question ... but it was bumped, so ...)
Both the question and the given answer from @cbh have misunderstood the actual quoted text that's the context of the question.
As pointed out in the comment by @whitecap in this context 'opposed' is not fulfilling the function of a verb - therefore the title of the question ("oppose", active or passive voice?) doesn't really make sense. In addition to the link in Whitecap's comment see this link which has some more information.
In terms of the OP's question whether 'were so opposed' should be replaced by 'so opposed' -- that would not stop opposed from functioning as an adjective. However it would not work in the way it has been suggested, instead the replacement would have to change 'were so opposed to' to 'so opposed'.
So the following two constructs would work:
They were so opposed to slavery that ...
They so opposed slavery that ...
In both constructs the word 'opposed' functions as an adjective and therefore the use of active or passive voice is irrelevant.
(The OP may have been confused by the use of 'were' which is not indicating a passive voice, but rather an imperfect tense (ongoing state at a point of time in the past)).