The most celebrated pin-up girl and model of this period was certainly the red-haired Evelyn Nesbit, whose career as a model, celebrity and focus of a huge scandal does indeed suggest that being known, in essence, for one's sexuality could easily attract censure and opprobrium in the early 1900s.

Nesbit, by the way, looked like this.

It's important to stress, in what follows, that she was in no realistic sense directly comparable to a modern porn star. She always posed fully clothed (so she and her mother insisted, though there are two partially nude portraits, alleged to show her, in existence), and never with men; her trademark look was girlish innocence. However, the parallels are there. Nesbit's poses were considered more suggestive and more alluring than was typically the case in that period; in addition she had an overbearing "momager" and she was extremely young – only 14 – when her photographic career began. She was also at least as well known for associating with celebrities as she was for her modelling work.

Nebit came from a relatively poor background in Pennsylvania – her father was dead, and she and her mother were in debt, leading her to accept work that was done for disreputable outlets (tobacco cards), and was, as mentioned, at least suggestive in nature – she would go as far as this – as a way of escaping poverty. She was also attracted to (or perhaps vulnerable to the attentions of) older, highly wealthy men. She embarked on an affair with a distinguished, flamboyant, and relatively elderly (he was in his late 40s) architect, Stanford White – the designer of Madison Square Garden – who called her "Kittens." She appeared at parties he held, playing – and showing her legs – on a red velvet swing; again, something considered highly provocative by more conservative types at this time. Nesbit lost her virginity to White after an evening in which she was plied with champagne until she passed out and was raped by him. White continued to see her and offer her financial assistance until she turned 17, after which his interest in her cooled.

In 1905, Nesbit formed an attachment with a much younger millionaire, Harry K. Thaw, who had fallen in love with her after seeing her onstage four or five years earlier. Thaw discovered the circumstances of her encounters with White, and thereafter treated her in a rather schizophrenic manner, alternately posing as a champion of her virtue and imprisoning her and violently sexually assaulting her. Eventually, he married her and, in June 1906, encountered Stanford White at a performance of the Broadway show Mam'zelle Champagne. During the finale, Shaw Thaw confronted White and shot him three times at close range, killing him instantly.

The resultant murder trial led to detailed evidence being given in court of Nesbit's relationships with both men. Unsurprisingly, given the time and the place, Nesbit's character became as much the subject of the trial as her husband's murder of her former lover; her career never fully recovered. She lived on into the late 1960s, latterly working as a potter. Her biography, American Eve by Paula Uruburu of Hofstra University, is a good place to look for additional commentary.

Even before the 1900s it was scandalous. Most of the very early photos of nudes were assumed to be prostitutes. It was even bad to be a photographer to take such pictures. If you'll notice that most of the photos from the late 19th century have the photographer's name and location somewhere, either at the bottom of the picture or stamped on the back. Photos of nudes don't have that because the studios didn't want to get a bad reputation or get in legal trouble in those areas that had laws against indecency. Without that information tying a photo to a particular studio in existence during the period makes it difficult to peg down an exact time period for some of these photos. That's why sometimes you'll see 19th century nudes with descriptions like "due to the clothing style (most were partial nudes) and hairstyle, this photograph is likely of a Civil War era prostitute."

Source: me :). An Evening with Venus: Prostitution During the American Civil War.

/u/mikedash has a super interesting response, but I'd be very curious to know more about 40's and 50's era pinup girls if anyone happens to know. In my mind that's the most iconic era for pinups.