The meaning of marriage has been in relation to realtionships between men and women for centuries, PD. It is only within the last 40 or so years that any society has allowed marriage between 2 people of the same sex.
For centuries there were other restrictions on the nature of the couples who were allowed to marry (in this and other countries) - e.g. not married before, even if divorced, restrictions on class or race of people allowed to marry.
There have been many cases before where government has chosen to extend the scope of those who can benefit from marriage - did you think all of those were redefinitions?
Marriage has been a religious ceremony far longer than it has been a civil one insofar as most socoeties used to be religious as much as civil.
No it hasn't - we've been through this at length before. Most of the ancient civilisations that we have good information about (Greek, Roman, Egyptian) saw marriage as fundamentally a civil legal contract - sure they might have been ritualistic religious elements in the ceremony but the key was the civil contract between those getting married. Indeed the early christian church wasn't really interested in the concept of marriage at all and left it to the prevailing legacy Roman civil structure.
As a result, a secular government can only change definitions to a degree.
It can change married as it sees fit, within its democratic mandate, and indeed is the only organisation that can do so in the UK. Don't forget that there is no valid marriage unless that marriage is conducted in accordance with the civil law. If you 'marry' in some religious ceremony that doesn't align with the civil law you aren't married.