The 27-year-old is on trial in the High Court at Auckland, charged with murdering Ms Millane on the night of 1 and 2 December last year.
It is not disputed that the British backpacker, who was about to celebrate her 22nd birthday, died in his apartment or that he later buried her body in a suitcase in the Waitākere Ranges.
The Crown's case is that the accused strangled her to death, while the man's defence says her death was accidental after they engaged in consensual choking during sex.
This morning the defence called Professor Clarissa Smith, of the University of Sunderland, to give evidence via audio visual link.
Prof Smith is a researcher of sexual cultures whose work has focused on women's issues in pornography, sexual entertainment and sexual practices.
The court heard she had been asked by the accused's defence team to speak about modern male and female sexuality, and BDSM practices, breath play and erotic asphyxia.
Prof Smith told the court attitudes about sexuality and sex were constantly changing and had moved on from "a powerful mythology" that sex had a single purpose.
"So it's not just reserved for one's life partner or marriage but it can be extremely pleasurable for people who are not in a long-term relationship."
She said there had been a rise in social media apps that connected young people with others who had mutual interests in the same locale.
"Young people talk about sex as forms of entertainment, self-realisation and forms of community. It's actually an incredibly important part of youth cultures."
Professor Smith said BDSM was an "umbrella term" for a range of sexual practices including bondage, domination and sadomasochism.
Prof Smith said the practices were about exploring the experiences of humiliation and violence in a "safe environment" without genuine intent to humiliate or be violent.
"Within the BDSM community it's widely accepted that there are rules of obligation and partnerships; certainly long-term couples will discuss what the parameters are for the activities, how far the scene might go and safe words.
"To people on the outside these things might not look interesting or pleasant but for those on the inside it certainly is."
Prof Smith said public discussions of "kinks" had become widespread after the popular novel 50 Shades of Grey was published.
Mr Mansfield also asked the professor if breath play - a practice whereby breath is restricted during sex - was aggressive or an assault.
"It would be if it's without consent but where two people have consented to take part in that then it absolutely is not. But that's the same for any kind of sexual act. Within BDSM it is very important that consent is given and that a couple agree on what they're going to do."
Throughout the trial the jury has heard evidence both the defendant and Ms Millane had an interest in choking during rough sex.
They met through the dating app Tinder but the jurors have also heard Ms Millane had accounts on BDSM social networks.
Yesterday, the court heard Ms Millane told a friend she enjoyed rough sex, while a former sexual partner said they had researched and experimented BDSM practices, including choking.
Under cross-examination by Crown prosecutor Robin McCoubrey, Prof Smith told the court while some people liked to be choked to the point of losing consciousness, it was not common.
Mr McCoubrey said the court had heard Ms Millane told a friend she would not engage with BDSM practices with people she had only just met through dating apps.
Ms Smith said researching safe practice was "absolutely" a good thing and trust was a central part to any BDSM activity.
"The idea of the power play for many people is about the ability to trust somebody enough that this is an activity you want to engage in," she said.
Mr Mansfield later asked Ms Smith what she meant when she said safety "should" always be a priority when practising BDSM.
She said sometimes people got carried away and this was more likely to occur in a casual context rather than a long-term relationship where safety was discussed.
"Sometimes the desire for someone will override someone's sensible head and taking a risk feels like it's got a reward at the end of it."
The trial before Justice Moore and a jury of seven women and five men is set down for the month but it is expected to finish earlier, with the defence likely to close its case today before closing addresses are delivered and Justice Moore sums the case up.
