Grown men didn't wear short sleeve shirts (unless they are doing manual labor outdoors in the middle of summer) until the middle of 20th century. This is an example of what men would have worn in the summer in 1900 to play sports in. Even shorts are rarely worn unless they are worn with high stockings.
Women's clothing (especially evening clothes) prior to around 1830's often displayed amble cleavage. It is often said that Queen Adelaide (who in turn influenced Queen Victoria) started the more conservative high neckline of women's clothes.
There were periods in both the 17th and 18th centuries where dresses were so low cut that nipples were exposed, as shown in these examples. Pictures and poems about these bared bosom fashions appear in broadsheets from the 1600's. Prior to the Victorian era, we encounter much less prudishness in fashion, as in literature and art.
I think the statement you present also discounts the importance of things like central heating and inexpensive fabrics in fashion choices. People with only a few outfits need to wear items that are practical, while the wealthy and people who live in a society where clothing is inexpensive can afford a wider range of styles. As well, the amount of coverage that is practical is quite different in a house with a modern furnace. The invention of synthetic fabrics has also changed the possibilities in terms of what designs people can wear, how much their clothing weighs, and what level of coverage is required.
If the argument was solely that women's clothing had become more revealing when it comes to skin exposure over just the last 150 years, there could be a little merit to it. However, 200 years ago women's clothing was not terribly more revealing than much of what we wear today. Around 1814 many styles had a low cut neck-line and short sleeves, but with a longer style skirt. Almost exactly like the popular maxi dress today. Go further back, as Srace mentions, and "modesty" is about not dressing overly ostentatiously, not about showing the figure. But, even in the more conservative mid and late 19th century evening gowns continued to expose a great deal of neckline, shoulders, and arms. The length of a ladies hem depending more on the formality and function of the garment. Shorter skirts were used in laboring situations. While you wouldn't see something shorter than the knee likely, the coverage of skin was more for protection from the elements. Things like sun, wind, and the dirt that was produced by industrial workplaces could do horrible things to the skin if not protected. The big change for women's clothing came in the late 1910s when women had shown through their industry in WWI that they too could be part of the work place. Just like the adoption of trousers for women working in WWII, this first time around no doubt had a great effect on practical (and safe) hemlines.
As for men, if you look only at their suits, coverage hasn't really changed much. However, men today rarely wear full suits. Men working in trades or labor 200 years ago made concessions as well. Removing the coat and rolling up the shirt sleeves, unbuttoning the collar, like many of these men. Trousers were considered a working class garment, only beginning to be considered fashionable 200 years ago. In many cases a working man then would be covering just about as much as a typical man today in a t-shirt and jeans. Shorts without covering the lower legs with stockings is a bit more novel.
The biggest difference isn't so much in that the amount of skin considered acceptable has changed drastically, more so that the formality of our clothing has changed. It wasn't that long ago where women could wear shorts at the beach or in very casual circumstances, but wouldn't consider wearing them to the movies, church, or even lunch at a friends house. Same thing goes for men, it wasn't long ago a suit was normal work attire, anything less and they were laboring or sporting.
