They are not legaly married though, so technically its not illegal. It hasn't been outlawed to live with your girlfriend; however many there may be. It is the fact that he had sex with a female under the age of consent (18 in Utah) they are merely throwing the polygamy thing in because they don't approve. As far as I know though there is nothing they can really do about their multiple 'wifes'.
If there were we'd see a whole lot of people go down around here. There are several blatant polygamist colonies in my particular area.
If you don't want them to know, tell them you have herpies