This is a question I've had earmarked for a while and was hoping to come back to. It's an important question with quite a lot of cultural and background information. And I could've sworn I read a reference to it in K. Dhondup's The Water-Bird and Other Years but after going through it again, I realized I'd have to reread it thoroughly probably just to find a passing reference if anything at all.

As you can probably imagine, Tibetan sources typically didn't keep detailed notes of such things. I have yet to encounter a single reference to explicit homosexuality in Tibetan texts that I'm used to going over. That said, the Tibetan historical tradition is long and wide and just because I haven't found it doesn't mean it doesn't exist... it just kind of points to it not being so well-highlighted and well-studied.

That said, there are accounts of sexual activity in Buddhist monasteries. There's a whole tradition of male love in Japanese Buddhist temples, and an unfortunate trend of sexual abuse in Tibetan monastic traditions that unfortunately seems to be ongoing without a real solution in sight. Mix in the fact that the Tibetan government between 1642 and 1950 was entirely a monastic government, and the absolute ecclesiastical government of Tibet being run with a strong theme of homosexuality makes not just a lot of sense, but can almost be seen with a level of inevitability.

Back up: Buddhist monks live their lives from a monastic code of conduct called the Vinaya (Tib: 'dul ba). The Vinaya is filled with hundreds of rules and vows with how monks are supposed to conduct themselves inside and outside of the monastic grounds. The Buddha Shakyamuni himself is said to have laid down these rules as the Sangha grew and developed. In story form, usually a lay person would come to the Buddha saying, "I saw your monks doing X." The Buddha would meditate on whether or not monks should do X. And the stories usually end with a full prohibition on X. A lot is covered in the Vinaya: what kind, how many, and what color robes to wear, how to handle money, what times to eat, etc. Monks also apparently needed to be told not to have sex with humans, animals, prostitutes, each other, and yes, outercourse, oral, and anal counted.

Apparently, though, in the Buddha's day no one thought of intercrural sex, i.e. rubbing the penis in between tightly closed thighs to create friction. So it is technically "allowed" in the Vinaya. If one strictly follows the letter of the law.

Of course, there are intense debates over whether the letter of the Vinaya is ever followed. For example, monks in Southeast Asia interpret the Vinaya's restriction against "intoxicants" to include (vehemently) alcohol, but to exclude smoking, which many Theravada monks do regularly. Meanwhile in Bhutan, the Vajrayana monks would never smoke, as tobacco smoke is an impurity, but occassionally take part in certain chang (beer) rites that predate Tibetan Buddhism, and have been known to use alcohol medicinally in colder regions and temperatures. Similar relaxations occur for all sorts of reasons (the color and thickness of robes depends vastly on available materials and climate, for example).

If we really really get down to what it is that behaviorally separates a Buddhist monk from the society of lay followers, it's that they are peaceful and celibate. Indeed, the two crimes ("crime" may be too harsh, the two Vinaya infractions we should say) that could get a monk forcibly expelled from the Sangha are coitus and murder. For this very reason, Lang Darma's first actions to destroy the Sangha (in traditional historical accounts, modern scholarship has thrown doubt on them) was to marry monks and nuns together and then to force the monks into his army.

But as time goes on from the earliest days of the Tibetan Sangha, the situation only grew more complicated. The society, we should say, not necessarily the Vinaya itself. Many joined the Sangha for non-religious reasons (because their families were poor and giving a child to the monastery was easier than figuring out how to feed them, because at various times a monk-tax was issued that demanded every third son be given to the Sangha, etc.) and a more complex political structure developed. I try to avoid terms like "decadent" since they are so loaded and very clearly biased against the monastic tradition, but there were clearly individuals (and many) who had less religious motivations in the power structure they not only found themselves in, but were at the head of, no less.

Really, only the Gelug of the Dalai Lama (Tib: dGe-lugs) claim to be 100% celibate. The Nyingma (Tib: rNying-ma), Sakya (Tib: Sa-skya), and Kagyu (Tib: bKa'-rgyud) all have celibate and non-celibate categories in their traditions. The Nyingma, perhaps the least celibate of the bunch, oftentimes pass their temples, monasteries, and dzongs through a hereditary lineage, with Nyingma Lamas suggesting to their disciples at what point in their spiritual development it would be appropriate to take a wife/consort. The Nyingma are not at all and have never been a united lineage, more of a collection of pre-Late Diffusion traditions that trace their heritage back to Padmasambhava. The Sakya are at the other end of that scale, with a top-down lineage hierarchy that is passed down patrilineally, but often from Lama to his nephew, and after securing the inheritance, the head Lama makes sure to take his vow of celibacy. The Kagyu, with a lot of Nyingma strains in their tradition, follow this tradition similarly, but in more recent times have shifted to a more celibate/reincarnate style tradition. The state religion of Bhutan is Drukpa Kagyu (more on that soon).

The Gelug, having made a secure alliance with the Mongols starting in the 16th Century, ended up conquering Tibet and governing from Lhasa under an administration called "Ganden Phodrang." This government structure would dominate Tibetan politics for just over three centuries. The head of the structure was (practically or nominally) the Dalai Lama, but in his minority (which was often, esp. in the 19th Century) the Panchen Lama would take over. There were various Regents and governing bodies below the two top Gelugpa Lamas and after the Ganden Phodrang's initial development in the 1600s, there was a lot of politicking and games-of-thronesing behind the scenes. (Dhondup's three books are good Tibetan-perspective summaries, but check out the booklist for all sorts of goodies regarding the development of the Tibetan political structure, esp. Shakabpa, Van Schaik, Laird, and Smith) The Fifth Dalai Lama himself is suspected as having engaged in Tantric sexual practices (though allegedly no homosexuality was involved) and it's rumored that both his successor the Sixth Dalai Lama and his regent Sangye Gyatso were products of these rumored activities (with a lot of reasons for why neither could be, including just... geography). Still, the Sixth Dalai Lama refused to become a monk, citing his sexual appetite as his primary reason for doing so. And Lhazang Khaan's Queen, Tashi Tsering, murdered Sangye Gyatso when he refused her advances (also she defeated him in battle, so there was a lot going on).

So I think there's a lot to be said that Tibetan historians aren't necessarily shy to bring up when sexual politics are brought into the fold. But like European monarchic politics, sex is most relevant and important when it's related to familial relations, inheritance, and power. There's a similar history of sexual politics in Bhutan, which I've expounded at length about previously, also in the r/AskHistorians Podcast 065 but again, homosexuality makes a poor performance. Yet in modern times, intercrural sex has become a problem in Bhutanese monasteries, and elder monks have been found to be abusing younger monks, and questions have been arising as to how long this has been going on (see Schäfer: for a long time) how prevalent it has been, and worse yet, what could even be done to stop it and make sure it doesn't happen again?

Buddhist monasteries in Bhutan are self-governing. At the very worst, they are under the umbrella branch of Sangha law (i.e. it's a religious theocracy that exists side-by-side with Bhutan's Constitutional Monarchy). And only the Sangha can investigate and prosecute the crimes. The Bhutanese police and judicial system have no jurisdiction... and you can automatically see the problem. It's difficult for young boys who've been abused to accuse the judge, jury, and executioner of a crime against them. In fact, it's just asking for those abuses to be swept under the rug.

It's not hard for me to imagine, and (I'm working my way through a book on Schäfer myself right now as well) I'd expect that the situation in Tibet would even be more pronounced, since the allegedly celibate monastics with absolute reign of the Tibetan government at the time that Schäfer was walking around Tibet had absolute control of the Tibetan legal system at the time. And to add even another layer on it, the reforms and modernizations of the Thirteenth Dalai Lama had been undone and many reversed by the time that Schäfer visited Tibet, which certainly would have made these powerful Lamas running the Tibetan government even more bold and brazen in their activities without a central and disciplinarian authority to keep out the worst of these abuses. To say nothing of consenting adult relationships in the monasteries. But that's a topic deeply in need of further investigation to confirm or discuss in one way or another.