The mother of a mature student described as “Britain’s most prolific rapist” says she did not know he was gay.

Reynhard Sinaga was sentenced to life with a minimum of 30 years last week after being found guilty of attacking 48 men in Manchester. Videos he recorded on two iPhones suggest that he attacked at least 195 men while they lay comatose in his city centre flat, having spiked their drinks with a date rape drug.

Almost all were heterosexual, with Sinaga boasting to friends of “saving” straight men he picked up when they were drunk outside nightclubs. “SuperRey saves straight boys from their monstrous girlfriend,” he said in a WhatsApp message after raping a young student in January 2015, joking that his bedroom was a “discreet straight boys sanctuary”.

His mother, Normawati, said her family did not believe men can be gay, suggesting that she did not know her son was attracted to men. “We are a good Christian family who do not believe in homosexuality,” she told the Sunday Times. “He is my baby.”

Giving evidence at his fourth trial before Christmas, Sinaga, a 36-year-old from Indonesia, claimed that he was mistaken for a “ladyboy” by his victims, who were unwilling to admit that they had slept with a man. “It is not an easy thing to come out as gay,” he told the jury.

But most of his targets had no idea they had been raped until the police knocked on their doors years later, having found videos of them being abused on Sinaga’s phones. Only his final victim directly accused Sinaga of rape at the time, having woken up during the attack.

Normawati described visiting her son when he was on remand at HMP Manchester, the prison better known as Strangeways, and asking him why he recorded the attacks. He allegedly told her: “I like to do it. It was my documentary.”

In the interview, at the family’s home in Jakarta province, Normawati spoke of her son’s Christian upbringing and life of privilege, funded by the family’s company, which runs a palm oil plantation and refinery.

She described Sinaga as a “gentle boy” who “loved to bury himself” in a book. “He is the eldest of my four children. He didn’t really go out much,” she said. “He was more interested in studying.”

He studied architecture at the University of Indonesia and then went to a college in the US. He returned after a few months, complaining that the students were more interested in partying than studying, his mother claimed.

He arrived in the UK in 2007 on a student visa and studied for an masters in sociology. He then began a PhD in human geography at Leeds University, commuting to Yorkshire, but his thesis – Sexuality and Everyday Transnationalism among South Asian gay and bisexual men in Manchester – was rejected in August 2016. He was then granted time to correct the work and resubmit but had not done so by the time of his arrest on 2 June 2017.

His mother told the Sunday Times she begged him to return home to work for the family firm but that he refused, saying he wanted to “build a career as a lecturer. “He said Indonesia was not a good place to live for him and he felt comfortable living in Manchester.”