Sure, YouTube is the 900-pound gorilla of web video, but pornography has been an underground driver of video formats since 8mm film. Until the iPhone, Flash was the de facto standard for web video, including porn. iOS devices, however, offer only one avenue for porn (the web, since porn apps are not allowed in the App Store), and only one supported video format (H.264). You can bet that porn sites want to capture mobile porn customers (don't look so shocked, of course people want to watch porn on the go, and don't forget iPads). There has been plenty of time for petabytes of pornographic video to be delivered to iOS devices, probably starting mere minutes after the release of the original 2G iPhone.
Will porn sites start delivering WebM video? They tend to be run with an eye toward keeping costs down, so hosting multiple versions of the same video, and especially recoding existing video, probably isn't in the cards. The cost for MPEG-LA licensing may be prepaid (especially for whatever encoder they use), may be negligible, or they may simply ignore the licensing entirely and expect that any individual site is too small to be worth suing. I'd bet on porn video on the web quietly ignoring WebM and sticking with H.264 delivered directly or to a Flash player. Anyone savvy enough to insist on a Free/free codec is probably unwilling to pay for porn anyway, so they are hardly hurting their market.
Ultimately, Google can't force WebM on anyone. The only weapon they have to wield in this is YouTube (Chrome really isn't much of a weapon in the market at this point in time), and they'd alienate everyone using iOS, not to mention everyone with an Android phone too slow or old to run Flash, if they stopped delivering H.264. Would it change anyone's mind, or would the YouTube app on the iPhone be replaced with a Vimeo app? Supporting less rather than more has worked for no one but Apple, they only pull it off by making it up in other ways (e.g. polish), and even then they alienate a segment of the market as a result; Apple's high margins allows them to concentrate on a smaller market, whereas Google's need for eyeballs for ad revenue means they can't afford to alienate large groups of people.
Whether you believe that none of the (enforceable) MPEG-LA patents apply to WebM or not (I suspect some do), and whether you believe that there is something morally superior to WebM over H.264 (iffy), it's hard to believe that it has any chance of defeating H.264 in the market (I obviously don't). I'm prepared to be wrong, but I'm not holding my breath.
New York... when civilization falls apart, remember, we were way ahead of you. - David Letterman