Wife or girlfriend doesn't make a difference, and to imagine my girlfriend getting defeated and humiliated in fights is my basic recurring sexy fantasy in all kinds of variations, and I think there are many for whom that's how the turn-on works. For different people, there might be different psychological reasons for it -?
There's a definite split between fantasy and reality for me though:
While in a fantasy my girlfriend could have her face spat on, her body squeezed in bearhugs and headlocks until she passes out, be spanked until she pleads for mercy, and eventually be 'thrown away' into a garbage container by the other woman, IRL only perhaps a test of strength like arm wrestling would be okay for me but not much else. No matter what turns me on in a fantasy, IRL I wouldn't want the woman I love to even have her face slapped once, never mind anything else.