As others have pointed out - CULT. The Church of England doesn't impart all these ridiculous boundries on its members - but then again, it isn't a dangerous organisation which breaks up families. People who go to 'Church' do so because they (mostly) want to, not because they're afraid of the repurcussions.
The danger of a JW marrying a non-believer is that the WT will lose a member due to them having some sense talked into them.