Let's take a moment to ask the question: when you're looking at another woman with lustful intent, are you committing adultery? Actually, yes - and you are as guilty as if you had gone to bed with the woman you're lusting after. Christ himself clarified, in the Sermon on the Mount:
"You have heard that it was said, 'You shall not commit adultery.' But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart." [Matthew 5:27-28]In God's eyes, there is no such thing as "just looking" or "just browsing." This is because, even though your actions may say "I'm faithful to my wife," your heart does not. Deep down, you aren't faithful to your wife. Think of it this way: imagine if someone you work with is all smiles and compliments to you, and then you find out that they're actually going around behind your back and talking about how terrible you are and how much they really hate you. You would feel betrayed, wouldn't you? You would think they were a hypocrite, wouldn't you? Now let's return to the subject of adultery: you're telling you're wife you love her and she's the only one for you, but in you're heart you're lusting after other women and telling your buddies crude thoughts from your heart about said females. This latter scenario is no different than the former; although the subject is different, both involve deceit and disloyalty - one in regards to friendship, and one in regards to marriage.
When you marry a woman, you engage in a great bond with her. You two are united not only in regards to your lease or bank account, but in mind, body, and heart. Your heart should belong to hers, as her heart belongs to yours. Your body belongs to hers, just as her body belongs to you. You can't make love to any other woman except her; similarly, the object of your physical attraction should be no one else but her. When you bring attraction to another woman in your heart, you're violating that unity.
You're likewise inviting further temptation, and the possibility of physical adultery. People often say "just because you're on a diet doesn't mean you can't look on the menu," but as any personal trainer, diet coach, or just anyone who has struggled to lose weight will tell you, if you look at that menu long enough, eventually you're going to order something. Most adulteries begin because the unfaithful spouse entertained thoughts and permitted those thoughts to gain greater control of them. You entertain lustful thoughts of a particular type of woman, or seek after pornography to satisfy some kind of craving, and it will get worse, and it will, over time, affect your marriage. With your unity broken, you will have two authorities in your life to contend with, and you will have to sacrifice one of them. "No servant can serve two masters" (Lk 16:13a).
If you are looking at other women in lust, and fantasizing about them unrepentantly, and continuing to do it without concern for your spouse or your soul...then you are guilty of adultery, and stand before God as guilty as if you had gone to another woman's bed.
At this point in writing this, I can hear the classic contention of, "Oh, I guess you're just sooooo perfect!" No I'm not, actually. I struggle with lust every day. I have temptations in my heart to be disloyal to my wife like any other married man. I'm an adulterer. I'm a sinner. And I can offer nothing but my repentance to Christ, and ask that he, who is a high priest who is not "unable to sympathize with our weaknesses," but rather was "in every respect...tempted as we are, yet without sin" (He 4:15), forgive me. Christ is the high priest for those who believe, interceding before the Father, and has given himself for atonement once for all time (cf. He 7:27). If you are a believer, you have a chance to repent and turn to Christ, and he will prove to be the faithful high priest he is. If you aren't a believer, then God is granting you, while you still have breath to breathe, to repent of all your sins, and to turn and repent, and change your ways, and put your faith in Christ. Under no other name can you be saved (Acts 4:12), and there is no other way except under Christ the high priest that you can have complete atonement for your sins. Consider this carefully, and treat this day as if it were your last. There is no sin too great for Christ to forgive, for there is no greater savior than Christ. God bless.