for how long? like the first few months? I don't think you can judge the relationship back then by the level it is at now--because 6 years is a huge investment and without kids or money affecting the decision most people wouldn't stay with someone that long unless they had some genuine feelings.
personally if it was someone I was with for 6 years it's someone worth trying to work things out with. I'd never do it to my girlfriend, but I don't think it's something I could not get over if she did it to me.
if she did it again, though, I'd melt her in a tub of hydrofluoric acid and then use her to paint the house of the person she cheated on my with's house red.