From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger rejects the correspondent's argument that the woman's status as an omeres mutar (one who believed the act was permitted) exempts her from the rabbinic penalty, demonstrating from Rava in Makkos, the Maharik in Klal 84, and the Rashba that this defense does not apply absent an explicit sage's ruling. However, given the extreme pressing circumstances including danger to life, he allows possible reliance on the Behag's lenient position, conditioned on convening a beis din of three and other great rabbis, while maintaining that the child born of this union is a safek mamzer.
Source: Shu"t Rabbi Akiva Eiger §206