To be fair I'm finding that a cashless entry is becoming the norm at many sporting venues. I went to a "100" cricket match, Horse racing & two rugby matches where entry was either on-line booking or card at the entry point so Oxford's system is in keeping with the the progressive era we live in.
One place I went for Rugby was a 100% cash free stadium entry, food, Drink & club shop were all card only & no amount of begging would sway them to accept cash