Why isn't it statutory rape?

According to this NPR story, a sect called the Fundamentalist Latter Day Saints ("FLDS") has built and moved into a compound on 2,000 acres in the West Texas town of Eldorado. The story tells us that the FLDS's are Mormon polygamists, and that they are believed to have, amongst their many wives, teenage brides in the 13-15 year old range. Aside from finding the secretive compound creepy, Eldorado residents are apparently (and appropriately) especially disturbed by these teenage "brides." The sheriff says he's keeping on eye on the situation, but that there currently does not seem to be anything he can do. This surprised me because, if it is indeed true that there are 13-15 year olds sexually involved with older men, there's the real possibility of statutory rape, something most law enforcement agencies take seriously. The problem as I saw it based on the information in the radio story is that the Eldorado residents (including the sheriff) and the NPR reporter all seemed to believe that these 13-15 year olds were actual wives. They're not. Texas does not recognize polygamy. That means that an FLDS man's first wife is his legal wife. The rest of his purported wives have no legal standing. And to the extent one of those non-wives is 13 or 14 or 15, and the man is having sex with her, that's statutory rape. Am I wrong here? If I am, please set me right.