The United States was involved in World War II from December 1941 through August 1945, with United States fighting forces engaged in conflicts in Africa, Asia, Australia, and Europe. The United States though began expanding its military forces in 1940 with a new Selective Service Act and naval expansion. With the expansion of the United States military, followed by war-time deployments, the United States government was deeply concerned about the risk posed by sexually transmitted diseases/venereal diseases to its deployed troops. A particular threat was posed by the "dynamic duo" of syphilis and gonorrhea, which in the pre-penicillin period took considerable resources and time to treat.
To combat this threat the United States military undertook an aggressive training and propaganda campaign aimed at educating United States soldiers and sailors to avoid sexual contact with women. The overall tone of the campaign is well captured by the posters above, with an emphasis on targeting women as the key vector for infection. The United States military worked to emphasize to its enlisted personnel that infected women were something to be avoided and the best method to do so was to avoid all sexual contact entirely. Failing that, the United States military issued prophylactics to its soldiers with detailed instruction kits, along with post-sexual contact disinfection kits.
Beyond posters however the United States military also issued its soldiers with a standard pamphlet upon enlistment titled "Sex Hygiene and Venereal Disease" (link here) which both reflects the outlook of the times and includes messages aimed at protecting female chastity and purity by avoiding STDs.
Some choice quotes:
"It's perfectly normal for you to want to go with girls-just as normal as to get hungry or sleepy. Women have the same desire to go with me. This attraction for each other is due to the sex glands. It's what keeps the human race alive."
"Just because you have this desire is no reason why you must give in to it. Sex relations should be kept for marriage. Between people who aren't married they often lead to shame, sorrow, and disease. The public knows this so well that laws forbid sex relations between persons not married to each other. Good morals limit these sex relations to marriage."
My personal favorite though:
"You wouldn't like to thin that the girl you marry had been used by other men. Or that your sweetheart or sister was letting herself be used by someone. You feel a duty to protect her honor. If you want the girl you love and respect to keep her body pure and free from disease, you owe it to her to keep yours the same way. Nothing is more unfair than to expect her to control her normal desires while you give in to yours. Americans pride themselves on fair play. A good soldier plays fair. Will you?"
A combination of patriotism, sexism, and yet a tiny acknowledgment that women have sexual urges as well. It sums up the 1940s in the United States so well to my eye.
Apparently the "Black Widow" image was a favorite of artists making these posters. Give it a search sometime online, there are hundreds of images repeating the same themes. However I'll close with my favorite product of this period, a film simply titled "USS VD: The Ship of Shame" - it was a real film made in 1942 that has a very basic plot. A group of sailors enjoys shore leave and has a "good time" with some local prostitutes and they all catch an STD.
The ship sails and the ships medic has no end of trouble with sailors reporting in with illness and a general collapse of morale and the sailors ability to fulfill their duties. Eventually the destroyer meets up with a Japanese submarine. The sailors nearly fail in their duties due to sore penises and swollen testicles but fortunately American pluckiness wins the war!
Also the film apparently contains multiple montages of highly infected penises to help remind sailors watching the film what happens if you get laid in port.
Check out some non-infected penis moments from this gem on YouTube. (Link here)
Sources: Wikipedia entry on World War II and military sexual health education, Mother Jones article on World War II and STD posters, the War Departments World War II sexual hygiene booklet, IMDB entry and Bad Movies entry on the USS VD: Ship of Shame