I don't think the addition of a like button (which for a forum isn't really needed in my opinion) would mean people demanding a dislike button.
Facebook would never add a facility, as it would seriously affect their ability to appeal to advertisers - as well as causing all sorts of problems with users.
Like is positive, dislike is negative.. so I can't see anyone wanting it. If someone is genuinely unhappy with a post, I'd hope they'd say so and give a reason (or report it if it was suitably inappropriate).