You seem to be asking, “Can a believer who is absolutely determined not to commit fornication sleep in the same room with his girlfriend without succumbing to temptation?” To answer this, we need to consider the “deceitfulness of the FLESH and its power over us.”

The FLESH is “the SINFUL NATURE” that still dwells in the believer. Listen to what the Apostle Paul said of “the flesh” in Romans 7:14-20, “For we know that the law is spiritual, but I am carnal, sold under sin. For what I am doing, I do not understand, For WHAT I WILL TO DO, THAT I DO NOT PRACTICE; but WHAT I HATE, THAT I DO. If, then, I do what I will not to do, I agree with the law that it is good. But now, it is no longer I who do it, but SIN THAT DWELLS IN ME. For I know that in me, (that is, IN MY FLESH), nothing good dwells; for TO WILL IS PRESENT WITH ME, but how to perform what is good I do not find. For the good that I will to do, I do not do; but THE EVIL I WILL NOT TO DO, THAT I PRACTICE. Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me.”

The Apostle Paul shared the experience he had in trying to please God in his own power. Like the one you referred to, he had “an aggressive mindset and was determined not to sin,” but no matter how determined he was, he still failed. His will was strong, but “the FLESH” was stronger! Why did the FLESH gain the victory over Paul? Ah, because he was trying to gain the victory over the flesh through his own will; he had “confidence in himself” and thus “confidence in the flesh!” We are told in Philippians 3:3, “For we are the circumcision, who worship God in the Spirit, rejoice in Christ Jesus, and HAVE NO CONFIDENCE IN THE FLESH.” Whether we realize it or not, if we are trying to gain the victory over sin by our own will (i.e. mindset), we will fail time and time again. The flesh will deceive us into thinking we can overcome temptation, but we are being “deceived by the flesh.” Jeremiah 17:9 speaks to this, “The heart is deceitful above all things, and desperately wicked; who can know it? We see this illustrated in the Apostle Peter who said to the Lord Jesus, “Even if all are made to stumble because of You, I WILL NEVER BE MADE TO STUMBLE.” Jesus said to him, ‘Assuredly, I say to you that this night, before the rooster crows, you will deny Me three times.’ Peter said to Him, ‘Even if I have to die with You, I WILL NOT DENY YOU’” (Matthew 26:33-35). Peter had one of the most “aggressive mindsets” that we see in Scripture, yet he did not know his own heart. He did go on to deny the Lord three times (verses 69-75). He had to learn the same lesson as the Apostle Paul, that he will never win the battle over sin by trusting in his love for the Lord and his own determined will, for in doing so he is “trusting in the FLESH.”

You may be thinking, “So, are believers destined to lose the battle with sin?” Thankfully, the answer is, “No, we can indeed win the battle with sin, but “not in our own strength.” Paul went on to say in Romans 8:1-2, “There is therefore now no condemnation to those who are in Christ Jesus. For the law of the Spirit of life in Christ Jesus has MADE ME FREE FROM THE LAW OF SIN and death.” Paul had learned that “in my flesh nothing good dwells,” but then he learned that the Holy Spirit was dwelling in him (see verses 4-14) and could “set him free from the law of sin and death.” From that moment on Paul had “no confidence in the flesh” but he had total confidence in the indwelling Spirit. He could say with absolute confidence in the Spirit, “I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh” (Galatians 5:16). This does NOT mean the believer can put himself in a place of temptation and expect the indwelling Spirit to keep him from sin. Romans 13:14 says, “But put on the Lord Jesus Christ, and MAKE NO PROVISION FOR THE FLESH, TO FULFILL ITS LUSTS.” If we willingly place ourselves in a place of temptation we are “making provision for the flesh.” The Spirit will be grieved and He will not be free to set us free from sin. As we saw, we are to “walk in the Spirit,” which means we are to yield ourselves to His control so He can give us the victory over sin. See also Romans 6:15-22 with Ephesians 5:18.  (DO)  (611.1)