Because, Levi alleges, they slept with guards and fellow prisoners or – rather – used sexuality/their bodies as a barter chip for their survival.

So this gets us into a couple of complicated topics surroudning homosexuality and sexuality and the concentration camps as well as how things are remembered, who influences the remembering and that these things are heavily influenced by social discourses at various points in time.

There is a couple of crucial components to understand here. The first one is the details of the Nazi persecution of homosexuals. You, OP, write that "As far as I am aware the Nazis made concerted efforts to round up and exterminate homosexuals" – this does not exactly get to the core of the issue. Nazi German persecution policies towards homosexualy, more specifically, gay men is less similar to that towards Jews as is suggested here. While my colleague /u/kugelfang52 goes into this here, I have also written about this before here, here, and here. The short version is that persecution of homosexuals centered on German gay men, who behaved in ways that were regarded as socially especially egregious and/or got them them into conflict with law enforcement. Ruediger Lautmann: “Gay Prisoners in Concentration Camps as Compared to Jehovah’s Witnesses and Political Prisoners,” in Michael Berenbaum (ed.): A Mosaic of Victims: Non-Jews Persecuted and Murdered by the Nazis, (New York: New York University Press, 2000) 200-206 writes based on a comprehensive review that about 100.000 homosexuals were charged and imprisoned by the Nazis, of those about 15.000 ended up in concentration camps most of them charged as repeat offender criminals and about 3.000 survived until the end of the war. This is not intended to mkae light of the suffering of these men but at the same time it is a very different structure to their perseuction when compared to, say, Jews.

With most homosexual men being imporsioned in the camps as repeat offender criminals, they belonged to the mass of prisoners with a green triangel that was probably by far the most diverse prisoner groups next to so-called "asocials". Structurally, and this is the next important point, the SS in the camps would specifically put German prisoners from this group in charge of work details and other positions of power in the camps because what they could count on was a massive animosity to the more organized and often more homogenous prisoners with the red triangle, meaning political prisoners. Often social democrats and communists their organization structure and inner group solidarity of these prisoners continued into the camps. Hence, the SS would strcuturally pit this group against other prisoner groups, most often the green trinalge prisoners and make them vie for positions of power and influence within camp society.

Green triangle prisoners were part of social groups that continued to be stigmatized and discriminated against after the war. I mentioned above that gay prisoners of the camp were re-imprisoned by the Allies after the war if they hadn't "served their sentence". This lead to the situation where most of the commemoration of the camps and the descriptions of camp society originated with red triangle prisoners and others who had a tendency to paint the prisoners with the green triangle as allies of the SS guards, brutal beasts, and deviants. Eugen Kogon, one of the fathers of German political science and left-wing prisoner in the camps, published one of the first books about the camps mixing his scientifc expertise with his memories – he describes the green triangle prisoners as homosexuals rapists on par with the SS, doing their bidding by sexualy assaulting social democrats after being order to do so by the SS. Hell, he even writes that the only reasons the SS imprisoned homosexuals in the camps was to humiliate the political prisoners there.

That is, to put it mildly, problematic as a view and interpretation of history. It goes to show however that people imprisoned in the camps were not free of the prejudices of their time and homophobia was one of them. The charge Levi indirectly makes here and that shines through so much memory literature is essentially the same as Kogon – it paints sexual barter for survivial as a frivoulous and deviant practice of the morally corrupt when in reality it was but one of the strategies of survival in the camps. The difficulty that even historical scholars – such as Anna Hajkova, who has written extensively about this in Theresienstadt – struggle with is that we lack a narrative framework for such actions. Within the social system of the camps, the border of bartering sex for food with other prisoners and consensual relationsships were prone to blur in really complicated ways.

As Hajkova writes in Sexual Barter in times of genocide:

Yet romantic and sexual relation- ships in Theresienstadt were closely connected with the economy and so- cial hierarchy of the ghetto. The range of sexual barter was wide and varied, from bringing dainties to a virginal teenage girlfriend, to being a function- ary’s lover for protection from transports, to the straightforward exchange of intercourse for food. [...] Every society develops its own form of sexual economy, dependent on structural conditions, culture, and the male-to-female ratio; in order to do justice to an examination of sexual barter in a place, we need to understand this place and its rules in depth.

She goes on to explain that sexual barter "in its many subtle forms became so ingrained into the structure of society that people often did not acknowledge its existence, just as we do not necessarily pay attention to the myriad ongoing social contracts in our “normal” world." But this is for Theresienstadt and for hetereosexual sexual barter. In other contexts, and especially in camps that were segregated into men and women unlike Theresienstadt, the situation presents itself differently. There too, sexual barter became normalized but more specifically homosexual sexual barter, which at the same time could never become so normalized as heteresexual sexual barter in Theresienstadt because most people in the camps came from social contexts that viewed homoseuxality as deviant and because of that even wehn understanding sexuality as a spectrum rather than a fixed position, it's not something that was open for anybody to practice.

While there is more to explore in this higly complicated and contentious topic, the gist when it comes to your questions is: levi mentiones these men because there were prisoners in the camps who would use their attractiveness, their bodies as chips in a barter society in the camp and this stood out especially to those not partaking because it was a socially stigmatized activity and later often used to demonize groups that were perceived as hostile.