Richard Brooks, one of the union's vice presidents, said the practice of banning speakers under 'safe space' policy was akin to not inviting someone to a party.
The issue of free speech at universities has exploded in recent months following a series of high-profile interventions from the actor Stephen Fry, classicist Mary Beard, and with campaigns against speakers such as Germaine Greer.
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Controversial author Julie Bindel, appearing alongside Brooks on the BBC's Victoria Derbyshire programme, denounced those who claim her writing has led trans women to self harm.
Others in the audience blasted "middle-aged people" critical of democratic union decisions.
And despite the furore, a survey of 1,000 students for Monday's debate found 66% supported no platform policies.
“There was a house party and you just weren't invited”
Richard Brooks, NUS vice president union development, explained the union's stance, saying: "There are six organisations on the NUS no platform list.
"This is very different to a safe space policy which is based on the idea that every single person has freedom of speech and everyone has equal right to freedom of speech, however some people have more equal rights than others.
"We’re making sure marginalised groups get their views heard.
"Both of those policies are democratically decided. They are not a part of a wider debate around censorship.
"I think everyone has freedom of speech but people’s platforms are different."
Brooks went on to liken safe space policies to not inviting someone to a social occasion.
"There was a house party and you just weren't invited," he said.
“I am constantly described as being like Hitler”
Julie Bindel, the feminist author who has caused controversy with her description of trans women, was previously banned from appearing at Manchester University under safe space policy.
Bindel denied her writing was transphobic, before saying: "I am no platformed by committees within the NUS. I am constantly described as being like Hitler.
"It’s deeply offensive, but we don’t have the right not to be offended.
"Other students get fed up that I’m not invited.
"This is in the context where I go to universities that... allow pornographers, Muslim clerics who believe in stoning gays to death… who allow all sorts of human rights atrocities.
"Yet, I am banned by some of them, why?"
Presenter Victoria Derbyshire interjected: "We asked trans students to come along but they wouldn’t, they refused to be in the same room as you."
"There’s no such thing as a safe space," Bindel replied.
Naa Acquah, general secretary at Manchester University students’ union, responded to Bindel's comments.
She said: “We didn’t no platform Julie, we did not allow her to speak through the safe space policy."
“These students live in the real world, they are attacked, they are abused”
Acquah explained: "When you stand on the stage and say they should be there and we should challenge them: the power of that keynote speaker sitting on the stage behind that table [compared to] you as a student who they’re personally attacking.
"These students live in the real world, they are attacked, they are abused, on Twitter, on Facebook, through physical abuse, we are not taking it out of context.
"We are not living in a bubble.
"It really annoys me when you’ve got middle-aged people on Twitter and all these things saying we’re ridiculous, we’re stupid, actually that’s very insulting.
"We’re not stupid. You’re just saying this generation who have more progressive views than you suddenly, we don’t think the same way."
Responding to the criticism that there’s no such thing as safe space “in the real world”, Acquah said: "It’s ridiculous, we do live in the real world. We know there’s not a safe space. We’re trying to create what we want in our spaces."
Reaction to the debate provoked passionate a defence of both no platforming and safe space.
Yet others disagreed.
A survey for the BBC published on Monday found on a majority of students supported no-platform policy.
Some 66% of the more than 1,000 students surveyed said that no-platforming was acceptable in certain circumstances.
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This article was amended on 25 April to correct an error. Richard Brooks referred to safe space policy as being a kin to not inviting someone to a social occasion, not no platforming as previously stated.