As minhk says. Going through a marriage here is a trivial process, and only you, two witnesses and the civil celebrant (a solicitor) need to know about it (*) - you can have a "proper" wedding party later in whatever style you wish just without any legal significance.
Getting married also means that the partner on a Dependant Visa can also work if you wish without further paperwork, which they can't on an extended visitor's visa.
Just to be clear though, I know that HK does not recognise "common law marriage" (i.e. there have been no formalities"). I'm not actually sure whether they recognise "Civil Partnership" (between a man and a woman) where there is a formal agreement as equivalent to marriage. Does anyone have any first hand experience of this?
(*) Obviously plus anyone you then tell about it for your own benefit - Immigration to get the Dependant Visa, Inland Revenue for tax benefits perhaps...