Same with her father, Matthew Symonds. His journalist mother Anne Symonds became pregnant with him five years after divorcing her husband, Richard Symonds; Matthew's father was John Beavan, Baron Ardwick, co-founder of The Independent. Anne's own father was rumoured to be Asquith, the PM, who supported her mother financially rather than her mother's husband who died in WW1 when Anne was a baby.
With a family like that, marriage is probably just a means of tying herself thoroughly into the political elite, being able to network internationally, and being able to get money out of Boris for at least the next 18 years.