From: Rabbi Moshe Sofer
To: Rabbi Shlomo
The Chasam Sofer upholds the Rosh's ruling that a child born of a kohen and an unmarried niddah woman is not a chalal. He argues that REBY's derivation from 'eileh' is genuinely necessary, since conception requires completed intercourse rather than mere ha'ara'ah, and the Beis Shmuel's distinction must be understood accordingly. He further demonstrates that even the Rabbanan may exclude niddah from the categories producing chalalim, and resolves related difficulties in Tosfos and the Toras Kohanim.
Source: Shu"t Chasam Sofer EH I §138