We interact with people everyday, male AND female, and appreciate the care they put into their hairstyles, what they are wearing, that they have a great smile or pretty eyes, or a confident stride, and there is certainly nothing wrong with appreciating beauty. The problem comes when we start to wonder what's underneath that nice outfit. For all we know, that person might already be married to someone else and we would be guilty of coveting someone else's spouse and that's one of the big 10.
Sexual attraction is probably the thing most of us struggle with most. We might as well just admit it and do whatever we can to minimize the risks. I think that's at the core of what Jesus was saying. The vision of the person enters the eye, hits the mind, thoughts form and travel on down into the rest of the body and it's not long before the body says, "Hmmm...that sounds like a good time". He wants us to capture those thoughts by the Spirit and shut them down before it gets that far. And it ain't easy.