"Thou shalt not commit aldultery"
"But I say unto you, That whosoever (keeps) looking on a (married) woman to lust (overmastering desire, with an element of wrath and indignity; epithumia), hath already commited aldultery in his own heart."
"lust' is translated from the Greek word epithumia, and there is nothing in this word that can be desribed as being the appreciation of the physical form of the opposite sex in a dignified manner, no it describes the opposite, we might use this word to desribe the habit of 'stalking' with intent to master with wrath. This is niot an indicment against feeling sexually aroused or looking admiringly upon the opposite sex, that is perfectly natural. What Jesus is saying is that it is not right to act in an manner of seeking to control somebody without respect to their feelings and dignity.
It is Jesus explaining that evil desires lead to evil actions and if you are concieving this in your mind and heart, you are already breaking God's laws. It is not against God's law to feel physically attracted to a member of the opposite sex, but if you are not respecting them and your behaviour is such that you are seeking to control them purely to satify your own over-mastering passion and desire, then that is not right, that could be what is known today as 'stalking', or just lewd and crude unwanted behaviour, that is just not acceptable. See Jesus is always fullfilling the law by extending it to your hearts and minds, God is now writing His law in the heart, not in the stone, He wants it in your minds, no point if your actions and thoughts are betraying each other, they must be one.