Marital relations is forbidden on Yom Kippur.
Harchakos: All the laws of Harchakos that apply when ones wife is in the state of Niddah apply equally throughout the entire duration of Yom Kippur even if ones wife is pure. This is a safeguard to avoid marital relations.
Speaking with wife: One should refrain from excessive speech with one’s wife on Yom Kippur.
A woman whose Mikveh night falls on the night of Yom Kippur: A woman whose Mikveh night falls on Yom Kippur, it is forbidden for her to immerse on Yom Kippur, and rather she must postpone it until the next night. See Chapter 3 Halacha 11B for the full details of this matter.
 By Tishe Beav we rule that by day one may be lenient. Seemingly this is because by T.B. its only a Rabbinical prohibition and by day there is less of a suspicion that they will have relations.
 Does this apply even by a Niddah period. And if so, then why did Admur specifically mention it. And if not, then why is Yom Kippur more stringent?