I say, and with God's success: If both partners are willing, they have sinned and should seek forgiveness. If one is willing and the other is coerced, only the willing one has sinned. It is recommended to give a dinar in charity if it is at the beginning of the menstrual period, and half of it if it is at the end. The one who makes it permissible should atone. Refer to: Dhakhr al-Mutahilin, pp. 146-147. Ibn Abbas reported that the Prophet (peace be upon him) said: "If one of you has intercourse with his wife during her menstruation, he should give a dinar in charity, and if he has intercourse with her after she has seen the purity but has not yet bathed, he should give half a dinar in charity." This is found in Sunan al-Bayhaqi al-Kabir 1: 316, Sunan al-Nasa'i 5: 349, Sunan al-Darimi 1: 269, Sunan al-Tirmidhi 1: 243, al-Mu'jam al-Kabir 11: 403, Musnad Abu Ya'la 4: 321, Musnad Ibn al-Ja'd 1: 436, and others. And God knows best.