Muslims have created and come to believe numerous myths regarding marital relations. However, Islam imposes very few restrictions in this regard.
During Menstruation
Marital intercourse is prohibited during menstruation.
They ask you about menstruation. Say, "It is a source of discomfort; so, keep away from women during menstruation [do not engage in sexual intercourse]! Do not approach them until they are purified! Once they are purified, come to them as Allah has commanded you!" Indeed, Allah loves those who repent and loves those who purify themselves. (Holy Quran 2:222)
Anal intercourse is also strictly prohibited.
Prophet Muhammad (PBUH) stated that the man who engages in anal intercourse with his wife is cursed.
Source: Abu Dawood 1847
Apart from these, there are no restrictions on marital relations. Any claims—such as those attributed to Ali (RA)—that intercourse is forbidden on specific days or at specific times are merely myths.
Claims dictating that intercourse must occur in a specific manner, or that spouses must not look at each other's private parts, are all fabrications.
Allah does not conceal anything, nor does He forget. Had there been anything to be prohibited in marital relations, He Himself would have stated it.
Couples should heed the aforementioned teachings. May the Almighty God enable us to practice these Islamic teachings and attain success.