ANSWER: Under the Old Covenant in the Bible, a woman was considered "unclean" ceremonially during her monthly menses or period. Sex during this time was forbidden.
And if a woman has a discharge (menses, the KJV calls it "an issue"), and her discharge in her flesh is blood, she shall be in her impurity (set apart from others) seven days. And whoever touches her shall be unclean until sunset (evening, Leviticus 15:19, Holy Bible Faithful Version).
Not only were women "unclean" during this period but also the men they had intercourse with and ANYONE who came in contact with her discharge.
And everything that she lies on (when she is apart from her husband) in her impurity (when she is having a discharge) shall be unclean . . . And whoever touches her bed (it does not matter whether it is her husband or someone else) shall wash his clothes . . . And whoever touches anything . . . (will be considered) unclean until sunset.
24. And if any man (ignorantly) lies (e.g. has sex ) with her at all, and her impurity (her blood, etc. ) is on him, HE shall be unclean seven days (verses 20 - 22, 24).
If you do a little research, you will find there are quite a few contemporary researchers who have found that there are many sound medical reasons for abstaining from copulation during the time of a woman's menses.
You also asked in your Email about sex through the anus. This activity (between a man and a woman in marriage) is not directly prohibited in the Bible. What is clearly condemned, however, is having this kind of intimacy with a person of the same gender. The word of God says he hates intercourse between men (Leviticus 18:22, see also 20:13, 15 - 16 and Romans 1:24, 26 - 27).
Recent research has shown sex through the anus is very often the means of transmitting diseases such as AIDS and others. An infection from the bacteria normally found in the digestive tract, such as E-coil, can also occur and be spread.
E. coli urinary tract infection (UTIs) are quite common. Nearly all of the urinary infections which occur are caused by Escherichia coli, a bacterium very commonly found in an animal's lower intestine tract. These bacterium can cause a whole host of different kinds of infections, including stuff like sinusitis and pneumonia.
Since blood is the most common growth medium used in laboratories to grow bacteria, and the most common area where e-coli are present is in fecal material (that is why most water parks forbid babies still wearing diapers to enter the pool without wearing something more protective), one can readily see how having intercourse during a period would be dangerous for both participants. Additionally, having intercourse through the anus is the rough equivalent of notifying the e-coli bacteria that you are looking to become infected.
The WebMD site states that while some kinds E. coli are ultimately harmless, others can cause bloody diarrhea. In a select groups of people, according to the site, this bacterium can lead to anemia or even failure of one or both kidneys (as a precursor to dying). It is good when we understand that God's laws in the Bible, whether they discuss a woman's period or sex in general, are not given to punish us but for our benefit.