Not really. They are trained to pose in the show ring, and they do it all the time. Plus they work for carrots; much less than a human model charges. 
Those second-floor studios are a problem, though!
ah, finally something in my field. i’m a full time working artist. did the art school thing and took a massive interest in the dynamics of nudity in art.
to answer your question (all these answers so far a true, but there’s more):
male nudity was the standard for a long time starting out. the greeks worshiped nude men. in fact, one of the first portrayals of “realism” in art was the Kouros.
this persisted for a few centuries–they really liked the male form (and the depictions of a warrior of fighter as the embodiment of the perfect human specimen).
before that, you get non-realistic fertility goddesses and statues of that sort, but the first realistically portrayed artforms were men.
over time, the embodiment of beauty almost universally became the nude female. yes, in art school you learn to draw nude life-models because of anatomy. you learn the flow of the whole form rather than drawing each limb, head, torso piecemeal. you can tell the difference between a consummate draftsman of human form vs just some guy who draws people ok by the way the figure flows–either as a whole individual (when correct) or as a camel (collection of parts–incorrect).
that does require a firm understanding of anatomy and physiology. looking at Da Vinci’s drawings really show the difference in this (compared to, say, the Byzantine or Gothic eras).
that is just to get the technical renderings correct, tho.
the aesthetic is a whole different matter.
as i said, the female form overtook the male in prevalence by a wide margin and has held sway for centuries at this point. there’s a good book called History of Beauty by Umberto Eco (and the companion book On Ugliness) that really lay out aesthetics and viewpoints throughout history.
while it’s much more complicated than this thread could ever cover, some ideals about nudity are fairly simple. for example, the greeks and romans considered a single exposed breast symbolic of fidelity, stature, power, and so on and so forth. they believed the Amazons removed one breast to be better archers, so the single exposed breast can be traced to those roots.
the main aspect of nudity in art up until the 1800s is it almost always had some symbolic purpose. it was the nude Venus, fertility or the splendor of creation–or showed shame or youthful innocence (nudity is a huge symbol of both a complete lack of shame in a pure way–as a naked child–as well as a shameful, “cast-out-of-eden” portrayal of disgrace).
it wasn’t until Goya’s La maja Desnudda that nudity was just explored for the sake of itself. that is, it’s widely accepted as the first “nude for the sake of nude” portrayal of a women. you might even call it the first piece of erotic art.
there is, in the modern mind, little distinction between nudity and eroticism. this is why i took such an interest in the subject, because i see a clear distinction. part of that is the intention, but a larger part of that is puritanical Western sensibilities. i believe that’s the same series of factors that mitigate male nudity as being more taboo than female. just looks at movies–full frontal male vs full frontal female are very much not equal.
to a large extent, this distinction is arbitrary. Europeans are a lot more cavalier about nudity.
so. your question seems to have a bias in the tone–equating a nude figure model to erotic or pornographic junk on the net or in a brothel. it’s important to note there is, for maybe more liberal minded americans, a massive gulf between nudity and nakedness. never once did i get any kind of erotic pleasure from drawing a nude. i even had a very attractive model come on to me during a session and it was so far from my mind i believe i hurt her feelings when i explained the reality of the situation. it had nothing to do with her level of attractiveness–it’s just the context of the nudity and the mindset of the endeavor. there is no sexual joy. nor intention. it’s art. while there are a lot of erotic artists, the actual creation is not the sexual experience many outsiders imagine. it’s light, line and shadow. in many ways you might as well be drawing a lump of shaped clay, or a car or a building. you focus on the line and proportion and shadow and highlights, and you get lost in the rendering, in the detail, and it stops even being a “naked person” and starts being a reference.
i have photographed many beautiful, perfect female specimen for various projects–something nearly all my male friends equate to some kind of playboy mansion goldmine of hormonal bliss.
let me be clear: IT IS NOT EVEN ALMOST THE SAME. in fact, the amount of stress and concentration needed to either draw or photograph a nude model kills any semblance of eroticism about the event. it stresses me out so much so that i’ve actually deferred to hiring female photographers to capture the references i need as to avoid even being there at all. and i’m not prude–i have a pretty over active libido, i’d say. it’s just that there’s a strong distinction between eroticism and artistic nudity.
An anecdote: Some 50 years ago I was an art student at a major mid-western public university. Life drawing was part of the standard curriculum for undergraduates as well as graduate students. When a model didn’t show up the students took turns. I don’t remember that any of us striped off for gratuitous modeling although some of our fellows were nude models on a paying basis. My problem was not any of the life drawing courses but a sculpture class where the model (nude) was a young woman I’d had a tremendous crush on in high school. It took about two sessions before I could concentrate on the work.
There are nude models and then there are naked models, depends on lots of things, mostly emotional.
Incidently, the female life drawing models were nude but the males wore a jock strap.
This was also the case 30 years ago in a Massachusetts state university.
which makes it more sexual…or at least fetishistic.
The OP has no reaction to any of the comments here?