Show's opening night at Riverside Studios had already caused controversy for featuring some of the most explicit scenes ever seen on a British stage.
A man in the audience was given oral sex by one of the actresses, although the company, La Fura Dels Baus, later confirmed the man had been a "plant" who used a prosthesis to fake the sex act.
However, on next night during the performance a woman, who later gave her name as Mistress Poppy, 33, moved into the aisle and performed a sex act with a man.
After the show she insisted she had no connection with the theatre company and that she was a "professional dominatrix" and porn actress.
She said: "I've done this sort of thing before, in porn films and live sex shows, and I would have loved to go up on stage and have full sex there too."
Her sexual partner also denied he was taking part in a pre-planned exercise. The 29-year-old German man, who would only give his name as Michael, said: "Everyone in the audience wanted to do what I did."
A Home Office spokeswoman said: "It does not make a difference if it was a member of the cast or a member of the audience who was involved. Sex in public is an offence."
There were no early departures among the audience, who had paid £25 each, the London Evening Standard reports.
