I just enjoy wearing white nylon briefs. It makes me aroused, but nothing more than that. After arousal, I put them off and wear my normal shorts because it looks like a sin. Is it a sin or a psychological problem? Is it not normal?
What you are doing is pursuing sensuality, which is sometimes translated as lewdness or lust. "Let us behave properly as in the day, not in carousing and drunkenness, not in sexual promiscuity and sensuality, not in strife and jealousy" (Romans 13:13). It is considered a work of the flesh because its sole purpose is to gratify the feelings of the body. "Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality" (Galatians 5:19).
Sexual arousal is a part of life, but it carries a danger of keeping our focus on the world. "Do not love the world nor the things in the world. If anyone loves the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh and the lust of the eyes and the boastful pride of life, is not from the Father, but is from the world. The world is passing away, and also its lusts; but the one who does the will of God lives forever" (I John 2:15-17).
In your particular case, you are training your sexual desire to be expressed through an inanimate object. It is easy, especially for young men, to get locked on a thought or action if such thought or action brings about a strong feeling. If you think about it for just a moment, you will realize how silly that is. It will not prepare you properly for expressing love to your wife in the future. Rather than chasing after sexual feelings, get back to improving yourself in productive ways.