From the moment that these two fought each other in the Chūnin Selection Exams, I wanted them to procreate. It was made even better when I watched the anime version, because now I could hear them and I have a slightly sexual imagination. *hides all of the lemons in my library*
So, why do I ship them? Well, they're similar yet different. They're both intelligent, but Shikamaru really doesn't seem much into the physical part of battle strategies- except for when he was avenging Asuma. Naturally, I'd assume that he's the bottom when it comes to coitus because it seems to require less effort. Meanwhile, Temari is aggressive and assertive... It kinda seems like she prefers to be in control to me... So, she's almost definitely the top. (I'm a virgin, so I don't know if intercourse involves physical strain or whatever.)
ANYWAY, they balance each other's personalities quite nicely.