A good question. God created us to experience pleasure, but he limits that pleasure to within a range in which that pleasure is good for us. Food tastes good, and God gave us this for our enjoyment, but if we are gluttons it is bad for us. Gluttony is a sin because it is harmful to us. Every pleasure comes from God, but overindulging any pleasure can lead to addiction and is bad for us, so God calls overindulgence in that pleasure a sin–not because he is trying to hurt us, but because he loves us. Every addiction is an overindulgence in a kind of pleasure which, in smaller doses, and in ways that God does not describe as sinful, is a good thing. A small amount of wine is a pleasure (as the Bible notes), but drinking a few glasses of wine is indulgence, it causes drunkenness, and it is very harmful to us.
Perhaps the greatest pleasure of all is that of the sexual union between a man and a woman. God created this so that we can enjoy pleasure with the partner we are committed to for life. Any other use of this pleasure is harmful. That is how God sees it, and I agree with God. Pornography is abuse. Every form of pornography is abuse, whether it is abuse of a woman, a child or a man. It is disgusting to use the physical beauty as a means to abuse that person. To be involved in prostitution is abuse of the person. To have homosexual relations is a perverse misuse of sexual pleasure. You say (or you propose) that it is not a problem to lust after a woman. I believe that the woman would not agree with this at all. I will tell you this: women do not wear tight clothing so that you can lust after them (at least not usually). Do yourself a favor and ask a woman why they wear clothes that you find provocative. They will say that it makes them feel beautiful. Go ahead and ask a woman if she dresses this way so that you can think of her in a sexual way. She will say definitely not!
Noticing a lovely flower is not the same as looking at a woman and thinking about having sex with her. As far ask I know, there is no possible harm to me or to anyone else if I look at and admire a beautiful flower. If you admire the beauty of a woman and go only that far, this, too, is not sin. But to look at her and contemplate her private areas and to imagine sexual things is abuse of that woman, and it is sin. It objectifies the woman. Let me make this suggestion: Ask a woman if she minds if you undress her in your mind and think of her in a sexual way. Again, this is objectification and it is a form of abuse. That is what God says. Why? Because the sexual desire is designed for one thing–which is pleasure in a marriage. Masturbation, pornography and lust reduce pleasure in marriage. Many scientific studies have proved this to be a fact. That is why they are sinful. God did not give us sex so that we could go to hell!!!! That is so wrong! He gave it because sex between a husband and a wife is the most beautiful and intimate connection between human beings. You can read Song of Songs in the Old Testament to see this. It is a beautiful mystery which should not be spoiled by us committing lust or pornography or prostitution or sex outside of marriage. God calls these things sin because they are bad for us.
It is a shocking truth that many Christian men look at pornography. How terrible that is! I never ever look at pornography. I have totally renounced such sinful behavior as an abuse of the women who we lust after when we look at pornography. To be aroused by pornography is to be unfaithful to my wife, it is to abuse the person in the pornographic situation and it is an offense against God. It his harmful in many ways. The fact that many Christian men struggle with this sin is not evidence that it is not sinful, but evidence that the Church needs to do much more to call men to repent of their sinful behavior.
I hope this helps.