The word "generally" in there is another of those things which makes a statement true and trivial at the same time. For one thing, it depends on how you count the fences (When you have a fence about not being a gay male and another about not being a lesbian, does that count as one or two fences?)
A more reasonable interpretation is to take "generally" as a qualifier for how wide the support is for the fence rather than for how common such fences are among the population of all fences--that is, there aren't fences with wide support, the majority of whose supporters wish to cause harm. "Mandatory ghettoes" are indeed a counterexample to the statement when read that way.
Another month, another rationality quotes thread. The rules are: