Well if this is true, then its unquestionable they were mocking the last supper IMO.
Not necessarily, and IMO not even very likely.
Let's for the sake of argument assume that Barbara Butch had indeed intended (or more likely assumed an intent) to reference The Last Supper and, by extension, the Christan New Testament. Let's assume that she posted about "The Gay New Testament" and then changed her mind.
The most likely scenario, from my perspective, is that she had genuine pride of a message of inclusivity using Christian imagery. Until recent years that would be very typical of Christianity; inclusiveness used to be a highly valued virtue among Christians.
Then she learned better, perhaps from the press even. And having learned better, she naturally corrected her post.
It can happen and seems to be the most likely scenario to me.
If the piece isn't a forgery, that is.