The Respect MP was attacked by anti-rape campaigners and faced a tirade of angry messages on social networking sites.
Mr Assange is wanted in Sweden for sexual assault charges against two women. He is currently taking refuge in the Ecuadorian Embassy in London after being given political asylum by the country, but the UK has said he will not be afforded safe passage.
Mr Galloway said the women's claims – which Assange denies – are "totally unproven" and the Wikileaks founder has been "set up".
He claims one of the women invited Assange back to her flat, had consensual sex with him and then "woke up to him having sex with her again – something which can happen, you know".
The Bradford West MP, speaking in a 30 minute video blog last night, then added: "Some people believe that when you go to bed with somebody, take off your clothes, and have sex with them and then fall asleep, you're already in the sex game with them."
In his video, shot in black and white and called Good Night with George Galloway, the MP said: "Even taken at its worst, if the allegations made by these two women were true, 100% true, and even if a camera in the room captured them, they don't constitute rape. At least not rape as anyone with any sense can possibly recognise it. And somebody has to say this.
"I mean not everybody needs to be asked prior to each insertion. Some people believe that when you go to bed with somebody, take off your clothes, and have sex with them and then fall asleep, you're already in the sex game with them.
"It might be really bad manners not to have tapped her on the shoulder and said, 'do you mind if I do it again?'. It might be really sordid and bad sexual etiquette, but whatever else it is, it is not rape or you bankrupt the term rape of all meaning."
A spokeswoman for Rape Crisis said Mr Galloway was factually incorrect.
She said: "Having had consensual sex with a woman once does not give a man licence to then have sex with her again at any time and in any way he pleases and assume consent is given.
"By the same token, having had consensual sex with someone once does not mean a woman has forever forfeited her right to withdraw or refuse her consent to further sex with that person.
"Sex without consent is rape. Mr Galloway's description of such sexual violence as 'really bad manners' is offensive and deeply concerning."
A spokeswoman for the End Violence Against Women Coalition said: "The Assange case has revealed some very worrying attitudes to rape and sexual violence among many commentators as to what constitutes 'real rape'.
"Research shows the majority of victims never report to the police. The head of the CPS said in January that media reporting and commentary on sexual violence is contributing to low reporting rates - women and girls hear what is insinuated about rape and do not report. What is urgently needed is a public campaign to challenge attitudes to sexual violence."
A number of people took to Twitter to voice their disgust at the comments.
Former First Minister Lord Jack McConnell wrote: "George Galloway has lost the plot. The only people who should judge this Assange allegation are courts in Sweden."
Tory MP Louise Mensch also tweeted: "So ironic that George Galloway's political party is called 'Respect'. #feminism"
A supporter of Mr Assange last night sparked anger by appearing to name one of the alleged victims on BBC 2's Newsnight.
Craig Murray said: "I think incidents which are dubious themselves as to what has happened, and Julian Assange has denied the accusations against him, are being seized on as a political agenda.
"It's well worth people going online to discover what they can about the allegations, about how they were made, who made them, what the people who made them did afterwards, and look at what happened. Let us look at the conduct of these women. I'm going to say some things I know to be true."
Mr Murray named an alleged victim, claiming the name was in general circulation, but was challenged by host Gavin Esler.
Meanwhile, in the US, President Barack Obama called comments from Republican Missouri Senate candidate Todd Akin about rape "offensive" and "way out there".
Mr Akin, a member of the US House of Representatives, said in a TV interview on Sunday that women have biological defences to prevent pregnancy in cases of "legitimate rape," making legal abortion rights unnecessary.
He has since apologised for having "misspoken", but Mr Obama said: "The views expressed were offensive. Rape is rape and the idea that we should be parsing and qualifying and slicing what types of rape we are talking about doesn't make sense to the American people and certainly doesn't make sense to me."