In a sense, the Kinsey Reports are a myth: the "10 percent" number is passed on from person to person mostly without anybody reading the actual papers or thinking critically. Subsequent US studies have mostly been in the three to ten percent range, and there is probably oversampling from urban populations. There isn't really any rhetorical reason to stick to 10%: even the most conservative estimates put the US LGBT population in the >10^7 range, and (more importantly) the Kinsey Reports are vulnerable to criticism in a way that subsequent studies are not.
I doubt that Michelle Bachmann has any interest in such nuance. Still, an unthinking political adversary is no excuse for sloppily repeating archaic exploratory results when far more rigorous and relevant studies are available.