Discussed in Mean Girls, where Cady is the only girl at the party who dresses in a non-sexy Halloween costume. "In Girl-World, Halloween is the one time of year you can dress as an absolute slut and get away with it."
During the Stark Expo in Iron Man 2, there are some dancers in midriff-baring, high heeled Iron Man costumes.
April: I want another nurse. Ann: Well, there are none. We're stretched pretty thin right now. April: Then I want a janitor. They can do what you do, right? Ann:Yep, nurses and janitors are totally interchangeable. April: Except no one dresses up like a janitor when they want to be slutty.
Also referenced in a comment by Ann: "Halloween is my favorite holiday. It's just the best. And I don't have to work! Hey slutty teenage girls dressed as sexy kittens... pump your own stomachs this year!"
Bill Maher looooves playing with this trope, especially when he hosts "Arabian fashion shows."
While there isn't an actual Slayer uniform, this was brought up a few times. Xander is quite taken by Faith when she first shows up, causing his girlfriend to say she should dress as a Slayer. Ditto for Connor, causing Cordelia to complain about the fascination with Faith's leather pants.
Played straight when Buffy's boyfriend Riley Finn does something she approves of; she drops a hint of rewarding him sexually and adds, "There could be outfits..."
A clearer example of the trope is Willow's "sexy ghost" costume in "Halloween".
Adult Amy Pond's first appearance in Doctor Who was while wearing a sexy police uniform. When the Doctor queried why she was dressed as a fake cop, she said he'd been waking up and it was either that or the maid outfit. An older woman in the village later commented that she thought Amy was a nurse, to which Amy replied that she was that as well. She's a kissogram. Presumably, there are a few other outfits that weren't mentioned on-camera.
The second Romana's taste in clothes was basically these - the actress imagined Romana would just buy whatever was on offer at the alien equivalent of Portobello Road and dress in it even if it looked a bit eccentric. Sexy Catholic schoolgirl, sexy Victorian bathing costume, sexy Fourth Doctor cosplay, and the sexy flight attendant uniform that later became Tegan's Limited Wardrobe.
A Halloween episode of The Big Bang Theory had Penny in a "sexy cop" outfit. She insists it's "sexy cop" and not "slutty cop" because the "slutty cop" outfit was even skimpier.
Psych has a male example: McNab, who actually is a cop, was revealed to have a second job as a hotel stripper which had him wear a sexy cop uniform.
On the battlefield of the war between the hippies and the frat orcs, fighting on the side of the hippies may turn up battles against a Naughty Sorority Nurse. She is described as being drafted as a medic for the frat orcs because she showed up in a sexy nurse outfit, and she was only in THAT outfit because everyone already took the outfits for the sexy stewardess, the sexy pirate, the sexy referee, the sexy rollerskater, the sexy plumber, the sexy train conductor...
In the 2011 Halloween event, the sorority orcs turned their sorority house into a haunted house. While they have been turned into various monsters along the way, each one is described as wearing a sexy costume of a randomly generated, increasingly-esoteric profession. Such as a sexy astronaut, a sexy dentist, a sexy clothier, a sexy patent attorney, a sexy feminist blogger, a sexy chemical engineer, a sexy lexicographer, a sexy rat catcher…
The Halloween costumes released for 2013 are this trope played much straighter. Most of the men get more accurate costumes (along with an admittedly creative 'mummy' costume for Ryu Hayabusa that still masks his face), while all the women get some of the Sexist Whatever Outfits you can imagine. Interestingly enough, all the costumes have a 'home-made' or 'cheap' look to them; so unlike most other games where alternate costumes can become near-complete, 'natural'-looking reskins, these are quite obviously costumes.
In Misfile, Ash claims that girls use Halloween as an excuse to dress up as whores using these kind of costume. Emily's rebuttal is that it's the one time of year that women can dress up like that and not actually be accused of being whores, and doesn't think it's a fair accusation anyway. Given that she wore a sexy nurse outfit the previous year and most of the women at the following party wore something similar, though, Ash did kinda have a point.
The Nostalgia Chick's Worse Witch review begins with Nella dressed as "sexy Velma," Elisa as a "sexy Starbucks daytime manager" and the Chick as a "sexy 19th century Alpine laborer." She mutters something about a law being passed to mandate these sorts of costumes. The episode also ends with a song by The Nostalgia Critic called "Everyone's a Whore on Halloween", which addresses men acting this way too.
In The Nostalgia Critic's commentary on his The Cat in the Hat review, he mentions how the character of Evilina's costume was very difficult to create. Since Rachel was meant to be playing a child, they went to a costume shop and asked for the most childish costume they had, but everything in stock was a sexy version of everything. They found a compromise with part of a sexy Raggedy Anne (seriously, wtf?) and an inside-out poodle skirt.
Parodied with the Marzipan costume in the Homestar Runner short "Costume Commercial". It's a bikini printed with designs modeled on her ponytail and dress pattern. The announcer decides it's not quite ridiculous enough, so they throw in a devil tail and a hairband with Cat Girl ears on it. Neither of which is even the right color for the costume.
A few of the fan-made Halloween costume reviews have featured Strong Bad reacting to beautiful women in "sexy" fan costumes, either with disgust (in the case of a woman dressed as "hot Homestar" in 2005's "More Fan Costumes") or delight (in the case of a girl dressed like a employee of his fictional pizza joint, The Pizz, in "Fan Costumes '07").
On American Dad!, Francine complained about this very trope, and Stan assured her she could defy it by wearing a non-sexy nun outfit. She does, only to find that the "Cowardly Lion" outfit Stan is wearing is actually a "Sexy Lion" outfit - complete with fishnet hose instead of pants.
Tulio and Linda in Rio wear bird themed outfits to get into Carnival. Linda's outfit was basically a sparkly bikini with bird accessories.
Childrens Hospital parodies this by including "sexy chunk of drywall" among the other standards after a sorority bus flips over on Halloween.
This trope is also exploited to the max every February during the Wellington leg of the Rugby Sevens tournament.
One article on MSNBC in fact lamented this trend, because when the writer went to buy a Halloweeen costume for her prepubescent daughter, she literally could not find one that was not marketed as a "sexy" costume and more Stripperiffic than she was comfortable with.