I know it is normal to become attracted to girls, but when I'm with my friends and we see an attractive girl they just glance at her and resume the conversation. But I blatantly stare at the girl for a prolonged period of time. Just the mere site of a beautiful girl can get me thinking about sex. I often find myself day dreaming about anal, oral, and vaginal sex with women. I know it is wrong to masturbate, but I do it at least five times a week. I dream about girls, think about girls, fantasize about girls, and masturbate to girls doing things like working out, dancing or on porn sites, having sex. Is this something to be alarmed about or is it completely natural?
Yes, you should be alarmed because Satan has you snared by the sin of lust or lewdness. "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). You are doing the very thing that Jesus warned against, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28).
The reason it is sinful is that people will act, given the opportunity, the way their minds are focused. Sin doesn't begin when you put your penis where it doesn't belong, it begins when you dream of doing wrong. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, all these evil things come from within and defile a man" (Mark 7:21-23).
Masturbation, by itself, is not called a sin by God (see: Is masturbation sinful or not?), but you are using it sinful as an outlet for your lust. Likewise pornography is equally sinful since its sole purpose is to generate lust (see: A Look at Pornography).
You don't have to be this way. You can exercise self-control.