This type of lust goes for women/girls also, and Jesus said:
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. I don't think it means that a person has done the act, but it does have the awareness of the act before it happen.
There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
1 Corinthians 10:13
The above is the reason why each party male and female must keep his/Jesus commandments. John 14:15
everything has its place, lust for the flesh has its place, that is acceptable in God's sight, which so happens to be between spouses. the desire of for has its place, no matter what it is. one must keep in mind that when God finished His creation He Himself declared it good very good, therefore anything other than what God made a thing for isn't.
as far as what the Lord Jesus is talking about, simple if you entertain it you have done it in your heart. (one could say imagined the act). therefore one is guilty already because its in his heart to do so. one should also understand that no one can escape what is in their heart. the desire of the heart will seek the opportunity to fulfill what is in it. hence when the Lord writes His laws in one's heart then the fulfillment of the law is one's desire for one's self.
hence,the mind of Christ,and what is in the Heart of God should be with those who are born of the Holy Spirit. considering that it is the Life of Christ that we are to receive in the son ship that Jesus enjoys, therefore, there is no reason that it should be thought of otherwise.