Well, the source cited is an academic study by Margaret Gonzalez-Perez called Women and Terrorism: Female Activity in Domestic and International Terror Groups.

I've looked at the section of the book that covers the female members of the Weather Underground. Gonzalez-Perez makes it clear that the "Women's Brigade," so called, was not very effective; it was supposedly 70-strong (FBI documents suggest this is a considerable exaggeration), but carried out only a single attack and "WUO women participated only at the lowest level of support." She notes that, not very surprisingly, "the very fact that women were joining terrorist organizations, in contrast to their gender stereotype, may have contributed to some degree of journalistic sensationalism..." and my sense is that this may help to explain how this claim came to be made.

Gonzalez-Perez's source for the claim is "Grathwohl, 1976: 149". Grathwohl is Larry Grathwohl, an FBI informant who was the author of Bringing Down America: An FBI Informer with the Weathermen. He lived with members of the group in Cincinnati for one year in 1968-69 and published his first-person book on his experiences in 1976.

So that's where the claim comes from. There does not seem to be any doubt that Grathwohl was a member of the Weathermen and he was taken seriously by his handlers (if only because he was the FBI's only informant within Weather for the duration of the group's existence); he did give sworn testimony to a Senate enquiry into domestic terrorism. But since Grathwohl was unique - the only outsider to report from inside the group – we don't know much about how the claim came to be made in the book, how accurate it was, or the degree to which Grathwohl exaggerated or glamorised – or indeed politicised, given that in later life he was prominent as a speaker to conservative groups – his time with Weather. There are signs that the publishers wanted to boost the appeal of the book by making sensational claims. They assigned Grathwohl a ghostwriter, and their blurb makes mention of "cult-like obedience" and "enforced sleeping arrangements"; the intro to the 2013 reissue of the book describes Grathwohl witnessing "joyless acts of 'monogamy-crushing' sex" – which, even if true, shows significant judgement being applied somewhere along the line. And we know that other members of the Weather Underground hotly disputed a number of his claims about their activities and ideas. Certainly I can find no evidence that this claim is based on any document - it seems to be just what Grathwohl reported about the behaviour of the Weather group he was a member of. He was too low level to be part of the senior leadership or to have known much about other Weather cells, so I think that it is safest to assume that, at most, what he claimed might possibly have applied to one part of this very disorganised organisation, for one year of its existence.

But there are further reasons to think the information may be sensationalised, misrepresented, represent nothing more than Grathwohl's interpretation of what he saw, or perhaps even be simply invented. Bryan Burrough's Days of Rage, which includes the most complete recent history of the Weathermen based on fresh research and interviews, does not consider the claim worthy of mention, and the material Burrough has on the Women's Brigade make it seem unlikely it is true, at least for the Weather organisation as a whole.

The main reason for supposing this is the case is that there is evidence of dissent from within the group; there were disagreements between the sexes which seem to have arisen at least in part because male members were pressuring female members for sex. The very fact there were disputes does suggest that, at minimum, not every member of Weather had signed up to the doctrine that Grathwohl claims existed in Cincinnati, and we can say with a high degree of certainty that female members of the Weather group did not universally subscribe to the idea that they were required have sex with male members on demand. Indeed, from what we know of the female members of Weather, a significant number were vocal and confident, and self-identified as "radical feminists". They were very unlikely indeed to have subscribed to such ideas or obeyed such instructions.

Here's Burrough's main passage on the female Weather people:

For a group as aimless as Weather, dissension was probably inevitable. As the months wore on with little sense of new direction, complaints bubbled up from the ranks, especially from those few dozen members who hadn't taken active roles in the bombings or leadership. Some were simply tired of living near the poverty line. A handful of gays and lesbians griped that the leadership wasn't attentive to their needs. The loudest complaints, though, emanated from several young women who began to self-identify as radical feminists. The Central Committee discouraged them from joining outside feminist groups, prompting gripes that the leadership had been corrupted by 'white male superiority.' Efforts were made to placate these women. They were allowed to form a 'women's brigade' within the organization that bombed a Department of Health, Education & Welfare building in San Francisco in March 1974. At one point it issued an entire white paper on the importance of feminism, not that it helped; one male Weatherman recalls a "Weatherwomen's" conclave in Marin County whose members emerged to insist that no male Weatherman could even talk with a female Weatherman without asking permission. This grousing culminated in a 1973 article in Ms. magazine by none other than Jane Alpert, who... announced that she was rejecting the underground because of 'sexist' leadership that 'exploited' women.

What all this suggests, I think, is not some general acceptance of "revolutionary sex" as a way of freeing Weather members from bourgeois conformity and morality – of sticking it to The Man, as Grathwohl's claims imply – but rather a more conventional pattern of male pressure for sex, exerted within the rather stifling confines of a closed, politically radical group at significant risk from the authorities, and of female unhappiness at, and in some cases at least, refusal to comply with, such demands.