Demonisation, if it occurs, is not the result of having sex during menstruation but of disobeying Yahweh's commandment. This gives demons associated with sexual impurity the legal right to either enter or harass without such a couple knowing it and removes whatever divine protection is given to those previously chaste and pure. Such people, especially those claiming the Name of Christ, are going to have their thoughts and feelings influenced by demonic powers if they do not repent of this sin.
The Fall of Adam and Eve occurred on the same basis. The act of eating of the fruit of the Knowledge of Good and Evil was less important than that of disobeying Yahweh's commandment not to do so because it gave Satan direct access to the couple once they did. There were, of course, other reasons for not eating the fruit at that time, but the act of eating was not the doorway to Demonisation and the Fall. The ground issue was one of ownership: to whom did Adam and Eve belong? They chose Satan by rebelling against Yahweh's authority.
There is a divine, loving purpose for abstinence during menstruation that has, in part, to do with the electrochemistry of the body. But the only ground for Demonisation is disobedience of the commandment itself. It is therefore a grave error to suppose that a woman is possessed of demons when she is menstruating and that when a man has sex with her the demons are simply being passed on to her -- an error that derives from the (correct) understanding that if a pure person has sex with one who is impure, that the demons infesting the impure one pass to the pure one (thus making him or her impure) through intercourse. And if you think about this silly scenario logically, it implies that the woman is sinning for no fault of her own, for to be possessed, influenced or controlled by demons in any sense implies sin on the part of that person. Such a teaching smacks of some of the more extreme doctrines of Original Sin.
If people would stick to the Scriptures and think clearly none of these foolish teachings would ever appear. As it is it would seem to be that this is just another one of Satan's attempts to denigrate women and to smear Yahweh's holy Name.
One final comment whilst we are on the issue. We are sometimes asked if there is any qualitative difference between intercourse during menstruation and in the one week period afterwards, and how much, if any, sex can take place then.
There has been a division of opinion over this question, some saying that "intercourse" refers only to proper sexual penetration, others to any kind of sexual activity that includes kissing and petting. The Patriarchate has made no judgement on this issue, leaving it to the leadings of the Ruach haQodesh (Holy Spirit) in those concerned, beyond what is said below.
Where God has not spoken there is obviously considerable freedom. However, that does not mean we can do whatever we please. Every born-again believer has the responsibility to discover for him- or herself both what Yahweh expects generally of all believers as well as what He expects for you personally. It is almost certainly true that apart from what has already been clearly revealed in the commandments, the way married people conduct their sexual lives will vary enormously, and that this will depend on (a) their natural dispositions (some have higher hormonal levels than others), i.e. their physical needs (real or imagined), and (b) their spiritual needs.
The key question is one of balance. Some peoples' lives are far too steered by sex, and others clearly have a lack of it because of mental or spiritual blockages of one sort or an other. An important characteristic of a disciple will always be self-control. There are some who are not mature in sexual matters who, once aroused, feel they just have to go "all the way" and can't help themselves. Such ought to remain completely celibate during menstruation and the seven-day period afterwards. If you are a young Christian, or weak in areas of sex, then my apostolic council would be complete abstinence during the menstruation period, and strict curbs in the week afterwards. If you have good self control, then agree with your spouse on what you feel is best for you. We do believe that complete abstinence during menstruation is desirable if only to give married people time for prayer. This might be an excellent time to do just that as Paul recommends:
"Do not deprive each other [of sex] except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again [for sex] so that Satan will not tempt you [to break the commandments] because of your lack of self-control" (1 Cor.7:5, NIV).
I can think of no better advice. Couples must work out their sexual lives sensibly and learn to be led by mutual consent and by the Ruach haQodesh (Holy Spirit). Where the lines are drawn clearly, do not cross; where there is a grey zone, tread carefully and wisely, remembering both your partner's needs (which may not be the same as yours) and the ultimate purpose of all relationships, whether marriage or otherwise, viz. to give Yahweh glory. If one or both couples are psychologically or spiritually damaged and such causes problems with their sex life, then they should seek counsel and, if necessary, have deliverance ministry to sort the problems out. Don't bury problems away and thereby create stresses and potential spiritual dangers in a marriage. Get the issues resolved in prayer and sound council.
Many people who react to abstinence during menstruation do so because of one problem or another that has not been properly resolved. In our over-sexed western culture, sex has virtually become an idol and people believe that the Lord has no right "interfering" in their sex lives by restricting them in any way. What restrictions there are exist only for man's blessing and to improve his sex life, not impede it. If there is a violent reaction to Yahweh's sexual laws then there is more often than not a demon - or a cluster of demons - behind it who wishes to guard his defiled space. You should be aware of this. Far from being out-of-date, Yahweh's sex laws are timely and of great importance to the smooth walk of the Christian disciple. Don't let anyone tell you otherwise.