What is the meaning behind Tsukihi's pose 17 minutes into in episode 1 of Tsukimonogatari?
Yeah, what Tsukihi said was 都条例 tojourei, which is the short name of the "Tokyo Metropolitan Ordinance Regarding the Healthy Development of Youths" (also called "Bill 156"). This ordinance underwent a major revision in 2010 that substantially expanded its scope. In particular, it required publishers located in Tokyo Metropolis to self-regulate and prevent the publication of "manga, anime, and other images (except for real-life photography)" that "unjustifiably glorify or exaggerate" certain sexual or pseudo sexual acts. 

This bill was obviously targeted at the anime/manga/etc. industry (given the exemption for real-life photography). And, of course, the industry (which is largely based in Tokyo) was not a fan of this - this ordinance has been a hot-button topic for some time now. I'm pretty sure that the whole ending of the Kenzen Robo Daimidaler anime was a reaction to it. 

In practice, the law has not been very strongly enforced. As of mid-2012, no works had been actually deemed "harmful", though I believe a small number (single digits) were deemed "harmful" in 2013. But anyway:


  What is the meaning of Tsukihi's pose here?


Glorifying a pseudo-sexual act, I guess. The pose isn't a reference to anything in particular; the only previous examples of it I was able to find were a few things on pixiv (one, two; NSFW) doing drawings based on the description in the light novel (chapter 004; keep in mind that the light novel was written in Sept 2012, nearly two years after the revised ordinance was passed). More generally, this is a pretty prototypical example of the kind of thing that could have been subject to scrutiny or outright banned had the "non-existent youths" provision of the bill gone through.


  And why did Koyomi freak out like that?


Well, here is his internal monologue immediately after Tsukihi does her pose:


  Don't poke fun at it!
  
  Panicking, I dumped the washbowl on her. The soap was washed away. I think that made it worse, at least as far as the Metropolitan Ordinance is concerned. But still, I was of the opinion that covering up bits and pieces was far less wholesome.
  
  Nudity is healthful and artistic.


I don't really know what to extract from that. Perhaps he was breaking the fourth wall and was worried that the government's censors would shut them down?