In many cases, women whose husbands are into porn will have their self images brutalized, because  he can't help but compare their bodies to the air-brushed, touched up, and made-up women that inhabit the internet. Worst of all, many husbands and wives never find the sexual satisfaction God intended in marriage because of the false smokescreen that is pornography. 

Maybe the reason that his sexual desire for you is waning is because he is using up his sexual energy sitting in front of the computer screen. By the way, the Bible calls this "defrauding your wife." I was talking about this topic with one of my counselor friends who specialized in sexual addiction. He related that the more involved a person is with porn, the more difficult it is for the porn addict to perform sexually with a real person.

The beauty of God-given intimacy is traded for a sordid fantasy that doesn't exist. 

To this point I have assumed that the one addicted is male. The truth is that the number of female sex addicts is increasing exponentially.

Now, let's move on to your second question: "What do I need to do?"

I am assuming that you are a Christian. I will answer you in that context.

Open up and bring the issue out into the open. Why is the person struggling with porn? Was he or she sexually abused? Did they begin at a young age and got hooked? Forever? Can they stop if they want to or is pornography a driving force in their life?

Please, don't fall into the trap of assuming that a partner who's looked at porn at one time or other needs to be divorced or can never make a good marriage partner. In my opinion, looking at porn is a sin like any other and needs repentance and confession to God for cleansing our very souls.

However, carefully consider that the burden of being shackled to a porn-addicted person can result in decades of misery and hurt. Hopefully, he will get help for his addiction. But, most don't. You can't force your boyfriend to change. You need to make a wise decision about continuing the relationship -- pursue counsel from people you trust, spend time in prayer, and go with the truth in your "knower."

Now, there is one more issue included in your question that we need to talk about: having sex with someone when you are not married.

According to the Bible, sex is for married couples. Sex is a holy thing and to be expressed only in a marriage setting. Sex with someone who is not your spouse defiles both your body and his. Sex outside of a marriage setting brings with it an ungodliness that breaks Jesus' heart ( 1 Corinthians 6:15-20).

Also, consider that Matthew 5:4 also comes into play in your life. Sex without marriage dirties up your spiritual soul and compromises your ability to both see God and to hear Him speak into your life. In one sense, sex outside of marriage will put a spiritual stain on your life.

One reason that marriage is a "big deal" to God has to do with the future success or failure of society. Families are the glue that hold a society intact. When marriages and families fail, society is on the way to destruction (1 Timothy 4:1-3).

I'd like to tell you to keep your eyes open and someday the perfect man who has never looked at porn will come along and you'll live happily ever after. Of course, we both know that man doesn't exist.

But, we do know that when we live pure and holy lives, surrendered to the plan and will of God, He is in a great position to bring just the right person along.

So, be honest and open with your boyfriend. Talk with him about becoming the kind of loving husband Paul described in Ephesians 5:25-31:

Husbands, love your wives, even as Christ also loved the church, and gave himself for it; that he might sanctify and cleanse ... not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish. ... So ought men to nourish, cherish and love their wives as their own bodies. ... For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.