He is also charged with assault against a student using a belt. He denies all the charges.
Defence counsel Phil Hamlin said the jury's task was to decide if the teens were "really consenting or not".
"You may not like it," he said, but added his client only engaged in sexual activity with willing women.
He said the students had in no way communicated to the artist that the physical relationships were wrong.
The onus was on the Crown to prove the teens were not consenting, Hamlin said.
He also said that "age is irrelevant", explaining that his client didn't believe any of the girls he was having sex with were under 16.
"Do you think that's reasonable? Because that's the defence he's relying on," he asked of the jury.
Hamlin said the charge of assault was a case of the artist role-playing with his student and having "Medieval sex" - there was no intent to harm.
One of the teens during the trial had described the artist's touching as "disgusting and inappropriate", but Hamlin said the teen's evidence had been "rewritten and redone to make it look indecent afterwards".
"There was nothing indecent done at the time," he said.
"His fate is in your hands ... He did believe, reasonably, that they were consenting at the time."
The artist, whose works are understood to be hanging in well-known galleries and offices, is accused of violating four women, aged 14-18.
He has said he never forced any of his students to have a sexual relationship with him during 2014 and 2015 at his private Auckland studio.