Abu Wa'il (Shaqiq bin Salamah) said: It used to be said that a person in a state of major ritual impurity (junub) and a menstruating woman cannot recite the Qur'an, nor can it be recited in the toilet, and there are two states in which a person cannot remember Allah: while defecating and during intercourse. However, when approaching his wife, he should mention the name of Allah before beginning the "act."
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1034