So, I'm hacking Apocalypse World, and have perused other hacks, and see that there's no analog in other hacks to the specials for the classes, i.e. the effects of sex, or they're just there. So what's the real purpose of Sex specials, and how are they supposed to drive the story. Everything else is supposed to drive it in some way, but at least in my games, sex with other PCs either (a) hasn't come up, (b) has been very forced/gratuitous, or (c) has been an uncomfortable subject. What narrative are the sex specials supposed to drive?
One of the potential outcomes of shared stress is intimacy. The sex special serves to indicate a fundamental aspect of character and provide a mechanic for a character's expression of intimacy.
Not in any mechanical sense, mind you, but the Sex specials provide one of the clearest possible constructions of how the character views trust and intimacy that I've seen in any system.
The Sex special serves to detail the role, and, shape interplayer communications. It is an explicit license for sex in game, and it provides structure and consequences around that fade to black.
In a stressful apocalyptic world, the mating-urge, subsumed or redirected or acted upon, is one of the most powerful drives. It can be a direction for character growth as two characters of players grow to trust one another (not a simple thing in this world) or it can be a direction for character growth as the two engage in dominance games or worse.
In a world about trauma, the Sex special reveals how the trauma of the world has impacted the characters. As per SevenSidedDie:
"The design goal is to prevent such an important aspect of human interaction from being consequence-free, reinforcing the intended themes of the game. It should also be emphasised that license doesn't make sex mandatory in a campaign – but when it happens it's a choice that has meaning. There's no such thing as empty sex in AW." (unless you're the BattleBabe)