Are Christians not permitted to have sex with wife when she's on her period? Acts 15:29 and 21:25 carry over four requirements from the old testament to apply to gentile Christians - one of which is abstaining from sexual immorality.  In Leviticus 20:18, it makes a big deal about having sex with a woman while she's on her period.


  If a man lies with a woman during her menstrual period and uncovers her nakedness, he has made naked her fountain, and she has uncovered the fountain of her blood. Both of them shall be cut off from among their people. (ESV)


However, Leviticus 15:24 makes it sound more like a ceremonial issue than a sexual immorality issue (which in that case may be a requirement which has passed away under the new covenant).


  And if any man lies with her and her menstrual impurity comes upon him, he shall be unclean seven days, and every bed on which he lies shall be unclean (ESV)


So the question is, does the Bible prohibit sex during menstruation between a man and his wife?
There is no external law that binds the Christian.


  1 Corinthians 6:12 "All things are lawful to me," but not all things
  do profit. "All things are lawful to me," but I will not be mastered
  by anything.
  
  Romans 7:4 Likewise, my brothers, you also have been put to death to the Law through the body of Christ, for you to belong to another, to the One having been raised out from the dead, so that we should bear fruit to God.
  
  Galatians 2:19 For through the Law I died to the Law that I might live to God.


So it is just a matter of sound judgement of what is appropriate and what benefits by the Holy Spirit in you. It is called by many names:


Law of Christ ( Galatians 6:2 ) 
Law of the Spirit ( Romans 8:2 ) 
My laws ( Hebrews 8:10, 10:16 )
Law of Faith ( Rom 3:27 ) 
Law of Liberty ( James 1:25 )


All aspects of the same truth pertaining to Christ's nature inside us partaken through heart transformation by the indwelling Holy Spirit also Known as Sanctification ( 2 Thes 2:13-14 )


  Galatians 5:6 ...All that matters is faith, expressed through love....
  
  We are now not under the law anymore, but instead we live by faith.
  And here is a shocker, something that will no doubt shut the mouths of
  those who still try to be justified by obeying the law. Lets look at 2
  verses first: 
  
  Gal 3:12 And the law is not of faith… 
  
  and also 
  
  Rom 14:23b … for whatsoever is not of faith is sin. 
  
  Can you see what it says when you combine these 2 verses? 
  
  Can you see it??? Since the law
  is not of faith, and since anything that is not of faith is sin, then
  it means that those who try to be justified by their own good works
  and try to live up to some moral code (the law) are actually living in
  sin!!
  
  From Christ the End of the Law
  
  Living by the Law is Separation
  
  The law presupposes the separate self to obey it. The fact that we see
  the law outside us as something we have to attain to, something which
  we are not now but should or ought to be, is our perpetual admission
  that we are lawbreakers and in separation from God. In this
  separation, we can only hear the law as if it is coming from outside
  us and therefore we are apart from it, so that we are bound to both
  the demands of the law and to the separateness in which to do them.
  
  This separate outer law demands that we live up to the reality that we
  are the image of God. That is an impossibility, because we are only an
  image, and not God Himself. Only God can be God. An image does not
  create or sustain itself, but is dependent on the object of which it
  is an image. We are images that have come out of the breath of God,
  and so we have some measure of life and will in ourselves – for we are
  persons in His image! As persons, we have all been tempted to try to
  become God ourselves by trying to be like Him, whether knowingly or
  unknowingly. It is the most absurd thought that ever came into the
  temporal universe. Still, we have all tried it. Like Lucifer, we have
  all said, “I will be like the Most High.” (Is 14:14).
  
  Nevertheless, the very attempt to emulate God, even if out of a
  supposed good motive – to do “good” – is the point of separation from
  God, because no separate will can work in Him. A will that says, “I
  will be like Him,” is already separated unto itself and not to God,
  because no created being can be like Him. He is Himself and there is
  no other. However, He has created us that through Christ in us we
  would be the visible expression of which He is the invisible reality,
  so that our lives are what they are supposed to be when we are one
  with God. Something that is one with something has no need to become
  or change into something else – to “be like” something, because it
  already IS a perfect expression of that with which it is “one.” It
  just is itself, and thinks and wills as one with its source. As a
  branch of a vine lives only from the sap from the vine, and therefore
  bears the fruit of the vine through its organic oneness with the vine,
  in the same exact way we bear the fruit of Christ since we are one
  with Him and are living branches on His vine.
  
  From Fred Pruitt


Further resources to study this are:


The Naked Gospel: The Truth You May Never Hear in Church
101 Lies Taught In Church Every Sunday
Pagan Christianity?: Exploring the Roots of Our Church Practices
The Story Behind Galatians - Frank Viola
Grace &amp; Law - Natural Church