I've just picked this up cheap in an effort to get into fighting games and had been really enjoying playing the CPU on the normal difficulty. I then made the mistake of playing my girlfriend and with no previous experience she beats me by spamming X & Y (no movement on the d-pad) every single time.
 
Is this my fault? Is the game borked? Should I give up and go back to RPG's?

It's low level soul caliber play's fault. (aka the game)

It's the game's fault. In no game should a first timer be able to beat a moderate-level player by just mashing buttons. The exact same thing happened to me with DoA4 on multiple occasions.

Most 3-D fighters have the same problem with button mashing at a lower level of play.   
 
Just invest more time in that game, eventually you'll know what do in situations like those (use longer range attacks, and guard impact specifically).

My guess is you let her win so she'd be spamming your buttons later on...well played, Sir.  Well played.

Happens to me with every fighting game.I beat the AI using actual moves,and think before using a move,and then a friend comes over smashing buttons and beats the everlasting shit out of me.  
Yeah,it really sucks.

I said it before and I'll say it again: Soul Calibur should have been made more like a Virtua Fighter with weapons.

Thanks - I feel much better.
 
Now I just need a strategy to convince her that I am the 'moral' victor.

Hello... Susie here... Aforementioned girlfriend. Felt the need to sign up for Giant Bomb and voice my side of the story. He lost because I am BETTER AT IT than he is. So there.
T'ra...
Susie

" Hello... Susie here... Aforementioned girlfriend. Felt the need to sign up for Giant Bomb and voice my side of the story. He lost because I am BETTER AT IT than he is. So there. T'ra... Susie "

Easy tiger.
 
Note to self - when asked 'what are you doing on the computer' - lie.