Subliminal virtue.

There's one good thing Cosmo does in every issue. They have a recurring feature, "Red Hot Reads," where they excerpt sex scenes from romance novels for your "his stalwart member plunged between her quivering petals"-reading pleasure. And in every single scene, there's something like this:

"Janey," he said, "I can't wait."
Janey felt the same way. All she could think was more. Jason slipped on a condom, and she guided him inside her. She lifted her hips to his... then pulled away. She lifted again, taking all he had to give her. Then he was grinding too, matching her rhythm. The sensation was so unbearably good. Rich, erotic, amazing. Soon he increased the pace.

It's never declared with fanfare, but the Red Hot Read always includes a condom. The condom is mentioned in passing and treated like a normal thing. It takes just one line--hell, half of a line--to mention, and although one instance of it doesn't do much, the preponderance of stories that include condoms in this way spread the idea that a condom is just a regular part of sex. It becomes part of the audience's model of how sex is supposed to go.

(Now I feel sorta bad that I don't often mention condoms myself. I use them absolutely every time; I just have a habit of considering them so normal that they go without saying, and my audience sufficiently sophisticated that you would understand that.)

In a way, this is more effective sex education than a straight-on screed about how you must use condoms because they're very important and you'll get horrible diseases and give your partner horrible diseases. Direct speech has its place in education--casual mentions in cheesy sex scenes won't tell you how to use a condom properly or what it's for--but sometimes fails as persuasion. Some people will use a condom because they're told it prevents AIDS and babies, but some will only use it when they're convinced that's what normal people do.

Now I just wish Cosmo would go further and model, say, consent in their stories.
"Janey," he said, "I can't wait."
"Neither can I," Janey said. All she could think was more.
A tiny difference, but it changes the heroine's reaction from lying there sending out consensual thought waves, to giving explicit enthusiastic consent.

Stealth propaganda isn't as much fun to write as direct screeds. It doesn't vent your spleen, and it can sometimes feel like a dishonest practice. It also raises questions of "do I want to perpetuate the beauty myth and heteronormativity in my consent-modeling romance novel, or do I want to risk losing my audience?" But it reaches people whom no degree of "Come on guys, you gotta wear a condom because it's really really important, listen to me" ever can.