LUST!!!

It seems to me that the Jesus warns us against interior lust as well as exterior actions of sin.

It also seems that God has given us a natural born sexual attraction to the opposite sex (at least in 97% of the cases).

Here is a question for discussion: What is lust? And how do we know the difference between lust and normal sexual feelings? Is there a difference? Is it always wrong to notice and appreciate the attractiveness of others? Or just when it includes lust? Should we feel guilty everytime we notice an attractive man or woman? Where do we draw the line?