I suggest not bringing her on as an employee or founder, but instead as just a consultant. She can come by and advise you with your code if you need it. You might even let her have a small piece of equity (non-controlling and certainly not more than any other founders).
Think of what will happen to your relationship if you have a fundamental disagreement with which direction your startup should go (and there will be a lot of course corrections along the way).
Worst case with a just-business co-founder: Your startup fails and you go your separate ways, sure that it's the other founder's fault that you didn't get the funding round.
Worst case with your girlfriend co-founder (or even simply employee): you disagree with her and piss her off somehow. She still works there so she goes behind your back and undoes what work you do, by sabotaging your efforts, and maybe even bringing other people in to your disagreement, and ultimately your relationship (not cheating, but forming factions against you within your startup). Now your startup ultimately fails as in case one, except in this case you've also lost your girlfriend.
Alternate worst case with your girlfriend co-founder: You have a fundamental disagreement with her, but because of the emotions involved in a relationship, she leaves the team entirely (and with no notice) taking her work with her (by acting maliciously or just simply leaving without letting other people know what she's done already, leaving them clueless as how to run what she was responsible for).
Either way, the downside is larger when your girlfriend is involved. Too many emotions involved in relationships to add to the stress of a startup. Don't do it.