For some reason it seem to be a trope in American movies, particularly comedies from around 1970-2000, to portray Swedish women as sexy, promiscuous, and somewhat dopey. It seems like a weird stereotype to hold about a straight-laced Lutheran country most Americans rarely think about. Where did this come from?
Many removed comments, but I'll give it a shot anyway.
What you describe sounds like what could be described by the "Swedish Sin", which I'll try to outline in a timeline of five movies:
Hon dansade en sommar (1951) (Eng: One Summer of Happiness) - It got a lot of credit when it came out, with a win at the film festival in Berlin, but it also got banned in a long list of countries for showing a nipple^(1); which at the time was much more chocking than it is today. It was as such a start of the idea of Sweden as a place from which "naughty" movies came.
Sommaren med Monika (1953) (Eng: Summer with Monica) - One of the most famous movies by famed Swedish director Ingmar Bergman. Also this movie contained a number of scenes that would be seen as tame today, but which at the time were chocking. However, it became even more chocking in the USA where it in 1955 where it was cut down to focus on the "naughty parts" and sold as "Monika: The Story of a Bad Girl" and it was in 1956 confiscated by Los Angeles vice for being "indecent"^(2). Finally, it is worth noting that at the same time as it gathered interest, in 1955, so did Time release a famous article named "Sin & Sweden", which did further help this image of Sweden^(3).
Jag är nyfiken (1967) (Eng: I Am Curious) - A movie that in 1969 was the 12th highest grossing movie in the USA and that until the 1990s was the highest ever grossing foreign movie in the USA^(4). Like the previous movies was it, for the time, a rather sexual movie. However, what is more interesting is that it in the case Byrne v Karalexis (1969 & 1971) ended up twice in the US Supreme Court, due to disputes over its obscene nature^(5). So what we have now is a very popular Swedish movie that is in the highest court in the USA, at the forefront of obscenity.
Svezia inferno e paradiso (1968) (Eng: Sweden: Heaven and Hell) - I cannot find how big this movie became in the USA, but it is the origin of the song "Mah nà mah nà" that you might know from The Muppets. Regardless, it is an Italian "Mondo film" and it exotifies Sweden as a place for free sex and largely continue the same idea of Sweden as a place of "sin", as such I think it can stand as a good example of a view that existed of Sweden globally at the time.
Maid in Sweden (1971) - Pure pornography, staring Britt Christina Lindberg who the year who apparently the year before had been the "Penthouse Pet" of the month^(6) and who in 1970 also was featured in Playboy (which I will not link due in order to avoid breaking some rule). Noticeable with the movie is that the producer, the director, and the financing of the movie all came from the USA^(7). The reason this movie came when it did was because the wave of pornography that was produced in Sweden at the time did force a legalization of pornography in Sweden early the same year^(8). It is as such a well financed porn movie produced by the USA, one year before the famous American movie Deep Throat and two years before Miller v California result in a liberalization in the USA itself, that was made in Sweden for practical reasons.
In conclusion, what you have if you look at this timeline of movies is a view in the USA (somewhat justified) of Sweden as a sexually liberal country. As a country in which young people, including young women, have sex because they want to and don't wait until marriage; as it says in the Time article from 1955. It built up during 20 years, it was referred to as the "Swedish Sin", and I think it is at least part of why you see that view of Swedish women in American movies in the 1970s (that then lives on).
1 https://www.svenskfilmdatabas.se/sv/Item/?type=film&itemid=4343
2 https://www.nytimes.com/2007/11/14/movies/14moni.html
3 https://content.time.com/time/subscriber/article/0,33009,861357-2,00.html
4 Ulf Jonas Björk, American Studies in Scandinavia page 113-134, "Tricky Film"
5 https://supreme.justia.com/cases/federal/us/396/976/ & https://supreme.justia.com/cases/federal/us/401/216/
6 https://web.archive.org/web/20071121025653/http://www.penthousepets.net/penthousepets.php
7 https://www.svenskfilmdatabas.se/en/item/?type=film&itemid=40362
8 https://fof.se/artikel/2011/3/den-svenska-porrexplosionen/
I think I can propose an answer to this, hopefully with enough detail. Please let me know if you’d like to know more!
Have you ever seen Taxi Driver? There’s a great/painfully awkward scene in that film where Robert De Niro’s Travis Bickle takes his date Betsy, played by Cybill Shepherd, to a see a movie. The movie? An exploitation film titled Swedish Marriage Manual. Betsy protests: “this is a dirty movie”. She’s right - in classic sexploitation form, this so-called “marriage manual” is porn in disguise to get around censorship rules in the US. By claiming an educational context, many films could get around US obscenity law. One famous real-world example of this sort of film was the 1967 Swedish film I Am Curious, but it was hardly the only Swedish sexploitation film being shown to American audiences.
More relaxed rules on sexual censorship in Sweden created an environment where production of erotic and even pornographic films occurred with US distributors then picking these titles up for domestic audiences. Even Swedish film’s greatest export, Ingmar Bergman, was initially introduced to American audiences through what could be termed as light exploitation work - his film Summer with Monika was distributed in the US as Monika: The Story of a Bad Girl in 1955, with the new title not leaving much to the imagination as to what the US distributor saw in this film (a sexploitation film that’d get butts in seats looking to see a naked lady).
In 1955, a few months before that rerelease of Summer of Monika, Time Magazine published an article that also helped lay the groundwork to this perception of Swedes as sexually promiscuous. In his short article, Joe David explores what he saw as an epidemic of unwed mothers, abortion, and sexual promiscuity. The impact was significant, and certainly a motivating factory in Monika’s release - after all, one thing exploitation films exploited was the public’s interest in current social topics and issues.
So, summing up - you have a mid-century American public reading articles about Sweden being sexually permissive, while also consuming an ever-increasing number of sexploitation and pornographic films sourced from Sweden that reinforced this notion to Americans. By the 1970s it becomes so ingrained in society that a stereotype is born.
Sources:
Weitzer, Ronald John (2000). Sex for Sale: Prostitution, Pornography, and the Sex Industry. Routledge.