IDK, considering everything Loras Tyrrell had to give up and swear to do, and the mutilation of his forehead.... I can easily IMAGINE that the high sparrow would have had Cersei confess to having sex with her brother even possibly accusing her of the truth, that all of her children were the spawn of her brother and not the King's.  If that had happened, Cersei's son Tommen would have been ousted from the throne immediately and I suspect a simple forehead mutilation would not have been enough to satiate the high sparrow's thirst for "righteousness", death for Cersei is the only possible penance for such crimes, I'm sure the high sparrow would have tossed in the murder of the king as well. Sorta recap the crimes of Loras Tyrrell's, compare it to the imaginable crimes/accusations for Cersei.... and the resulting punishment.  IMO  Cersei did what she had to in order to survive.. let alone prosper as she has done, first at the direction of her father and then on her own accord.   
"On the day of Cersei and Loras's trial, Margaery stands with her father in the Great Sept of Baelor, and watches from the sidelines as Loras renounces his allegiance to House Tyrell and confesses to his accusations. She comforts her father and restrains him from intervening when the Faith militant begin carving the seven-pointed star into Loras's forehead. When Cersei does not show up for her own trial, the High Sparrow sends Lancel Lannister to bring her to the sept by force, and after Lancel himself does not return after a prolonged period of time, Margaery suggests to the High Sparrow that Cersei's absence and that of her son indicates that she is plotting to harm them, urging him to evacuate everyone before it is too late. Clearly underestimating Cersei, the High Sparrow dismisses this notion and has his Sparrows keep the Tyrell siblings from leaving the sept, along with everyone else present. [27]   
Margaery Death 
Margaery gives one last look to the High Sparrow, knowing her fate is sealed by the hands of Cersei. 
Powerless to do anything, Margaery clutches her brother as the sept erupts in an explosion of green wildfire, killing everyone inside, including Margaery, Loras, and their father. As a result of Margaery's death, Tommen commits suicide after he sees the burning sept from his chambers in the Red Keep"     |