This kind of policy seems to have become more common in the last few years, requiring employees to notify their manager of any relationships that start at work. The idea behind it in most cases is to make management aware of possible conflicts of interest. If a staff member and their line manager become an item, for example, there is the potential for favouritism in promotion or performance reviews.
A blanket 'kiss and tell' policy isn't good HR practice though. It's a heavy-handed measure, which doesn't treat staff as responsible adults. There are also questions about how practical it is (e.g. at what stage should you declare it – on the first date, or after your engagement party?). Some couples may have their own personal reasons for keeping the relationship secret.
If you are dismissed for not revealing a relationship, you may have a case for unfair dismissal at an employment tribunal (though be sure to take proper advice promptly before beginning any legal action).
Even if you don't reveal a relationship, you won't lose your rights to protection from harassment if you then become a victim of harassment as a result of that relationship breaking up.