Whoa!! Hold on there break2012

You don't give much background information, but based on what you've mentioned your g/f does NOT need to leave UK to remain as the spouse.
There are no regulations preventing her from marrying you and then switching to a spouse visa via an application for FLR (Further Leave to Remain).

Any person granted given entry clearance or leave to remain in any capacity, for more than 6 months is able to switch to a spouse of a person present and settled in the UK.

The key issue is that when she will submit the application for spouse FLR the new-rules that will be implemented on July 9th will apply and you will need to comply with all of them.(unless you can marry and submit application before Friday July 6th)
Two key issues mentioned will be Income Thresholds and English Test pass certification.

These changes are still being understood by us all, but I'm pretty sure I understood that technically when FLR is applied for, the new income threshold for FLR can be reached by combined joint income.

Here's what UKBA said in their Statement of Intent
Income from employment or self-employment of the sponsor (and/or the applicant if they are in the UK with permission to work).
Hope this will help you and lessen the stress