When God created Adam and Eve He gave them their sexuality. Sexuality was not something the devil created, it was and still is Gods idea and originally created by Him for Adam and Eve to have sons and daughters of God. Its easy to make the misconception that God wanted them to have a dull and dreary sex life, almost sort of a duty something they had to endure to have children. When God created Adam and Eve and gave them their sexuality it was for pleasure and enjoyment first and foremost, and through that pleasure and enjoyment children would come as a result. So by reading Genesis 1 it becomes clear to us that God is the inventor of our sexuality and He wants us to experience sexual pleasure and enjoyment. 

 
We know what happened next, Adam and Eve fell from grace and lost their glory because of their choice to listen to the devil lies instead of continuing to trust God. As a result of the fall we see how this affects their sexuality. The sexual enjoyment and pleasure God had intended for them was soon warped into something evil and sinister where sex no longer became a source of joy. Instead it became a weapon of selfish desires and ambitions. (Gen 19:33)
 
The reason for the fall and the reason for lucifers revolt was egoism. Lucifer went to war against God because he became proud and wanted power. He tempted Adam and Eve to fall for his lies by tempting them to focus on them instead of God. When Adam and Eve accepted his lies their object of faith shifted from God and what He could do to them and what they wanted. This of course affected their sexuality. 
 
God is a God of joy, love and laughter. Sexual enjoyment and pleasure is a big thing to God. He wants us to enjoy and have pleasure in sex. He wants us to feel the arousal and pleasure when we are turned on by a person of the opposite sex. But He wants to protect our sexuality from the effects of the fall. He wants us to keep sex as a source of pleasure, enjoyment and love. To achieve this He introduced marriage. When one man and one woman comes together and promises to love, care and serve each other their commitment to each other ensures and secures a sexuality filled with pleasure, enjoyment and mutual love. 
 
If we look at society today we see the opposite of this, we see the effects everywhere and especially in all the perverted expressions of our sexuality. The homosexual lifestyle, pornography, rape, child abuse, free sex, cohabitation. They are all expressions of our sexuality being perverted by the fall. Gods judgement and wrath is not against sexual enjoyment and fulfillment in the marriage, it is against all of the perverted expressions of humankind sexuality. 
 
Marriage is the only place where we can experience our sexuality the way it was intended to be. Within the walls of marriage where a man and a woman comes together and promises to serve and love each other, they are free to experience sex the way God created it to be. Something for enjoyment, relaxation and pleasure built on mutual love, respect and promise to serve each other.
But we know not all marriages work out the way they where supposed to. Marriage is not a guarantee of eternal bliss and happiness. And sometimes, sex in marriage is not what it was intended by God. A successful marriage is built on the cross as the object of their faith and a successful sex life in marriage is built on the cross. 
 
Marriage as an institution is under constant attack from the physical world and the spiritual world. satan has a special hatred for marriage because God has chosen marriage as a symbol for His love towards humankind. The husband is called to love and serve his wife the way Jesus loves and serves us. The wife is called to love and serve her husband the way the church is called to love and serve Jesus. And if need be, the husband is called to die for his wife both physically and spiritually. (Gen 2:24, Matth 19:5, Eph 5:25,31) Marriage is supposed to be a testimony to the unsaved world of Gods love and Gods willingness to serve and lay down His life for an undeserving humanity. 
 
In marriage you have two individuals who are both struggling with their sin nature. The husband is called to lead and to serve his wife, but at the same time he has to deal with his sin nature. The wife is called to submit to her husband and to serve him. That way the needs, both in the bedroom and outside the bedroom of both the husband and wife will be fulfilled. This is not always the case simply because both individuals have their own struggles with the sin nature. But this is also why both individuals needs to submit to the cross by choosing the cross as the object of their faith. 
 
The husband will only be able to lead, guide, serve his wife, die from himself, and serve as the priest of the household if the cross of Christ continues to be the object of his fait. The wife will only be able to submit to her husband, serve him if the cross of Christ continues to be the object of her faith. Both their sexual needs will only be met if they both have the cross of Christ as the object of their faith. That is why satans biggest attack on christian marriages today is trying to make them change the object of their faith. By introducing marriage counseling, 12 step programs for a happy marriage and other man made fads he can try to tempt the husband and wife to leave the cross of the Christ as the object of their faith. If he succeeds in doing this he has succeeded in destroying the marriage in every way. 
 
If a marriage leaves the cross of Christ it has left its only spiritual, physical and emotional protection. 
 
The husband the wife have a joint responsibility in keeping their focus on the cross of Christ, but sometimes one of them will fail and be tempted to shift the focus to other things. When this happens, (and it will happen more then once) it will be the responsibility of the other spouse to maintain his or her focus on the cross of Christ. The other spouse must make sure he or she is not drawn into the confusion and continue to keep his or her focus on the cross while praying for the other spouse. 
 
When you leave the cross of Christ as the object of your faith the sin nature is awakened and things starts changing. All of the sudden it becomes about me and mine needs instead of serving the other. This can be challenging in a marriage, and this will happen more then once in every marriage with both spouses. When this happens it is important the other spouse maintains a steady focus on the cross of Christ and not be drawn into the confusion and chaos. Maintain your focus and not allow yourself to react from your own sin nature. 
 
When both spouses fights for their marriage and makes sure their focus is constantly on the cross of Christ the marriage is protected and blessed by God. In that kind of marriage both spouses will experience the joy and fulfillment of their sexuality. When troubles arise both spouses fights with the cross of Christ as the object of their faith, and when one of them falls the other one continues the fight (1.Tim 6:12)
 
But then something unexpected happens, your sexuality is not created with an off switch and it stays on. You are happily married, the marriage is under the protection of the cross, and both of you have the cross of Christ as the object of your faith. But then one day you are out walking and you see another person of the opposite sex and you are turned on by her/him. That person is very sexually attractive and before you know you find yourself imagining how it would be to see that person naked, or even how it would be to have sex with that person of the opposite sex. Then all of the sudden you find yourself struck with guilt, you are happily married. You should not do this, you should be faithful to your spouse and somewhere in your mind you remember Matth 5:28 and you are convinced  you have just committed adultery in your mind.
 
What we first of all need to remember is this, sin is wrong object of faith and not so much what we do or dont do. Of course it is a sin to lie, steal, committ adultery and worship idols. But we cant hinder ourself from sinning, we will always sin. So God has made it very easy for us and decided that He will make sure we will live a holy sanctified life and not lie, not steal and not break His 10 commandments if we choose to have a righteous faith. We can only have a righteous faith if our faith is focused on the cross of Christ.  But why then did God take the time to tell us in Matth 5:28 it is a sin to look at a woman with lust in your heart.? To show us our inability to live righteous by our own efforts and our desperate need to have a rightous faith. 
 
Second of all we need to understand Matth 5:28. Being turned on and sexually attracted to someobdy who is not your spouse is just the way you where created. God created you that way to be turned on and feel sexual attraction to an individual of the opposite sex and He never installed an on/off switch. So your sexuality is constantly switched on. So Jesus would never go against His own creation and tell us not to feel emotions He has given us in the first place. Matth 5:28 is all about the will.
 
Being sexually attracted to a person of the opposite sex is not a sin. Imagining how it would be like to have sex with that person is not a sin. Deciding to have sex with that person, but never going through with it, is a sin. But it is a sin that was dealt with at the cross. So the only way to have a natural relationship with your sexuality, where it stays on and you allow yourself to be sexually attracted to persons of the opposite sex, is through a constant faith in the cross of Christ. Having the cross as the object of your faith is the only protection against going to far sexually and ending up wanting to comitt adultery but not going through with it. And even if you where to fall and suddenly find yourself in a situation where you did comitt adultery of the mind there is still room at the cross. So if you have done that sin the cross is still there waiting for you to turn back to it so that Jesus can deal with that sin and take care of it for you. 
 
The cross of Christ as the object of our faith is the only solution for a happy marriage and a healthy sexuality