Sister Wendy Beckett's view is that it is a representation of what we have done to Christ today. I'm Catholic but that doesn't mean bringing out one nun that started studying art in her 50’s and is now called a “renowned art critic” changes that Piss Christ is trash. In fact pissing on anything has never seemed to me as representative of anything, it is a literal act with specific meaning.
I didn't trot out Sister Wendy to appease Catholics. I'm a fan of her meditations and spirituality so i reached for her interpretation. There are dozens of other highly religious critics that have tackled it if you consider hers substandard because she started public criticism late in life.
As well - you says it's a literal act with specific meaning. He literally did not urinate on it. The urine was carefully collected and lit a very specific way in an attempt to look beautiful. See above about literalism and single avenue interpretations.