Recently I have been getting urges to look at pictures of naked women and, even though I try to resist, it happens anyway. When I do look, I get erections, but I don't want to have sex. I guess I only do it for pleasure. Is this just arousal or is it a sin? The last time I did it, I prayed a lot and asked God to give me the strength to stop. I have not seen pictures like that since and the question was just filling my head.
"For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5).
"Passion of lust" refers to sensual things that are designed create lustful desires in you. We call these pornography. The people of the world chase after sensual things, but God's people know that getting pulled into the world will pull them way from God. "Therefore, since Christ suffered for us in the flesh, arm yourselves also with the same mind, for he who has suffered in the flesh has ceased from sin, that he no longer should live the rest of his time in the flesh for the lusts of men, but for the will of God. For we have spent enough of our past lifetime in doing the will of the Gentiles -- when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries" (I Peter 4:1-3).
Yes, your body reacts to sexual images, but it isn't good for you to chase after these things. The sin isn't in the fact that your body reacted, but in wanting to lust after women's bodies.