This may sound too simple, but the answer to this question is, because we are human, and therefore sinful (Psalm 51:5). Our hearts are deceptively wicked, even believers’ hearts, because we won’t be free of sin until glory (2 Corinthians 3:18) when Christians will have totally surrendered to Christ’s Lordship.
Strong sexual desire in and of itself is not a sin. For instance, a husband and his wife should have a strong sexual desire for one another (Song of Solomon, Ephesians 5:21). It’s part of a healthy marriage, yet it is not to be the only mutual desire. More often than not, marriages built solely on the shifting sand of lust and sexual desire fail.
Thank God for sanctification, which is a lifelong process. Yes, we are immediately justified when we recognize we are sinners, repent, and turn to the Lord in faith, and say “yes” to His kind invitation to salvation. We’re not “there” all at once, though. How could we be? It’s this author’s contention that we can’t accept full glory all at once because our sinful heads would “explode” with the heavenly weight (Job 42:3, Psalm 139:6, Proverbs 30:18). Our most gracious and merciful heavenly Father knows this. That’s why He allows us to grow more and more into the image of Christ, from one degree of glory to another (2 Corinthians 3:18), and at a pace that suits our unique selves. Some mature faster than others, and that’s okay if there is growth in the right direction.
But Christians indeed continue to struggle with sin, as Paul declares in Romans 7:14-25. He had a hard time doing the things he should and not doing the things he shouldn’t. It’s the same for us. Daily prayer, time in the Word, and regular gatherings with fellow Christians behooves our walk. Through these regular acts, we remember from where we have fallen (Revelation 2:5), and that we were at one time darkness, but we are now “light in the Lord,” and we are commanded to “walk as children of light” (Ephesians 5:8). Christians are no longer slaves to sin, and it’s not that we can’t sin, but we don’t have to sin anymore. We’re no longer held by the evil one’s power (darkness). We are not of the world any longer (John 17:16), and we are to glorify God in our body as we are transformed by the renewal of our minds (Romans 12:2).
Humans without the indwelling Holy Spirit, however, are slaves to the world’s values and its false wisdom (1 Corinthians 3:19, Galatians 4:3). They can’t not sin. The devil operates in the world under the guise of fun and fanciful feelings, with a desire for better things at the fore. But the better things from a worldly view are the things which keep a human in bondage to sin and evil, concupiscence gaining top billing.
The only answer to the question of how to eclipse the struggle lust is Jesus, the Way and the Truth and the Life (John 14:6). A Christian’s bondage to Jesus Christ is something the world will never understand—it is freedom (1 Corinthians 2:12-14).
The best we can do as Christians is to immerse ourselves in God’s Word and do what it says, which includes reaching out to the lost (Matthew 28:19-20).
Take that, devil!