Uni accommodation is for, you guessed it, uni students only. You also aren't allowed to live there because the contract is for one person only. I don't think you'd manage to do it without staff finding out about it which might end up with your girlfriend getting evicted halfway through the year, which would be very stressful and would likely have repercussions on her actual university work as well.
Also the rooms are tiny, even the large ones, and you'd be tripping over each other constantly and it'd just be way too cramped. You'd probably hate each other by the end of it!
Beck House is the only Swansea University accommodation that allows couples, but that's for postgrads and international students.