This is always one of the hardest subjects, both to deal with, and to define. "What is lust?" is a question that people have been asking for centuries.
Job said, "I made a covenant with my eyes not to look lustfully at a girl." Jesus said, "But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart."
In both cases, the adverb lustfully is used, but the bible doesn't define what that is. The strictest definition would say that we should not ever look at a female that causes feelings of attraction. The only way to do that would be to isolate ourselves from women entirely (except a wife). The other end of the spectrum would define lust a recurring obsession where you repeatedly imagine a sexual relationship with someone - anything else is fair game. I'm pretty sure neither one is what Jesus had in mind.
It's even more difficult to measure when it involves an image, photo, or video. Most would agree that watching hard core porn is wrong because it degrades women, men, and the holy act of sex. But what about artistic (non-sexual) nudity? If someone appreciates the aesthetics of a female body without a personal connection, is that lust? What about a clothed female on TV? If you have a crush on an actress on your favorite TV show, is that lust? What about a art? If you appreciate a painting with a nude female, is that lust even though it's not even a real person?
A famous quote form the US Supreme Court when trying to define pornography said, "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it".
I think that is probably true of lust as well... there is no objective definition. Romans 14 tells us that it is possible for something to be a sin for one person, but not for another. He gave the examples of drinking wine, eating meat, and celebrating the sabbath on a certain day. I think alcohol is a good parallel here. For some, drinking any alcohol is dangerous (and potentially sinful). For others (including Jesus), it is a normal part of life.
So it partially depends on how your mind works, but if you are married or dating, it also depends on the perspective of your mate. Here's an example: one of my best friends was married to woman who was extremely jealous and distrustful, and would flip out if he looked at another. Ironically, she ended up cheating on him. Another friend has a deep, trusting relationship with his wife, and they are "people watchers". His wife even points out girls that he would like when they are out.
In the end, there is very little specific biblical guidance. Even the subject of masturbation is notably left out of the Bible. Nearly all biblical scholars agree that the story of Onan was about coitus interruptus in defiance of God's will. I'm sure we'd all agree that it is often an addiction that can replace intimacy in the marriage or lead to worse thoughts and behaviors. But many Christians also support it in the right context. Perhaps that's why it was left unsaid.
Ultimately, we have to trust the Holy Spirit to help us discern what is destructive and sinful in our lives when it comes to this subject. If you rely too much on someone else's definition, you could err in being either too strict or too loose with it. Stay close to God and scripture and the answer will come.
One last thing (and I may get in trouble for this

). I think that women should be careful when making black and white statements about any of these subjects. The battle that rages with men is very different than that of women. To be dismissive of the different reality of sexual urges in men would be like a man trivializing the pains of child berth. Glad to have your input - we need it! Just be cautious.