From: Rabbi Moshe Sofer
To: Rabbi Pinchas Leib Munk
The Chasam Sofer argues that 'pitui ketanah ones hu' is a legal fiction — the minor's consent is treated as coercion for punishment purposes, not because she truly lacks free will. This resolves the Sanhedrin-Yevamos contradiction and defends the Rambam's view that a minor wife who willingly committed adultery is nonetheless forbidden to her husband, since exemption from punishment does not eliminate the prohibition created by her willful treachery.
Source: Shu"t Chasam Sofer EH II §172