So I can speak a little to the issue in the ancient world, particularly the Roman world which had much greater sophistication in its categorization of slaves and more prominent "pleasure slaves." To start with, while your concept is definitely right, I'm not familiar with the terms you use. Puellus is a form of the diminutive puerulus and doesn't seem to always carry a sexual or slave connotation. Though, just as in the antebellum South, the use of diminutives like calling a grown man "boy" was one way that those in power expressed their superiority over their social inferiors.

But you're right about special terminology, often these child sex slaves are called puer amatus (loved) or delicatus (pleasing). However, I've got two caveats on this terminology. First, it doesn't necessarily designate a sex slave. A delicatus could simply be a favored child raised in the master's house with status more akin to a pet or a foster child. Some of these children actually appear to be illegitimate children of their master who are raised with indulgence and then freed at maturity. Which is not to say that there weren't sex slaves, a good number of them definitely and explicitly were. Second caveat, these terms are also used in the feminine, so this isn't a gender exclusive category. So to be fair the Romans had little girl sex slaves too (yay equality?).

As to the preponderance of male representation (and it is always important to remember that just because something is talked about more it doesn't necessarily mean it is more common), there seem to be a number of factors that contribute to this. Ancient sexuality is a hella loaded thing to talk about, but if we limit our examination to the mere language than there is one obvious and interesting difference between them and us. Roman sexual vocabulary is not gendered, they talk much more in terms of act and active/passive than in terms of gender. It was the role of the adult male to be the active/pursuer/penetrative partner, but the passive/pursued/penetrated partner could be either male or female.

There is pretty reasonable evidence to suggest that homosexuality was in a way seen as a developmental stage in the Greek world. It was pretty widespread and even part of the political and social structure of cities to have an adolescent boy - even high status citizen boys - act as a "beloved" to an older man in the community. However, once the boy grew his beard he was meant to transition from the passive role to the active (and some men were pretty harshly criticized for plucking hair or imitating higher voices to remain in this passive role into adulthood).

The issue is, as an adult citizen male, you don't want to have played the role of the penetrated partner (and not to digress, but Greek art suggests mutual masturbation or intercrural sex to avoid putting citizen boys in a role that was socially looked down on). Plato's Symposium gives us some perspective on this as homosexual relationships are viewed as 1) being part of a male education and 2) more fulfilling than relationships with women who were treated as subhuman in the Greek world.

So to answer part of your question, homosexuality was acceptable, but it was constrained in different ways, homosexual sex with a peer was considered taboo, but with a social inferior, particularly one who was younger it was romanticized. Women were also used as sexual slaves from very young ages (in fact, it seems that the Greeks/Romans had an idea of 'adulescent' for men that they rarely applied to women who pretty much go from child to women with menarche). But Greeks and Romans had kind of a more "alpha male" mindset where being devoted to a woman was shameful. However, being obsessed with a young boy was okay. So there is definitely a greater representation of these relationships in our sources, even though our archaeological evidence (eg, from brothels) suggests there were as many or more girls in the sex trade.

I can't speak to the issue as it is represented in modern media, but I suspect that the combination of two salacious features - pedophilia and homosexuality - make these stories more appealing to the modern audience than similar stories about victimization of women which is perhaps so much more common as not to attract as much interest.