My understanding of impure thoughts is that they are rooted in lust. My understanding of lust is that it’s a sinful longing for someone. Here’s a definition from the New Catholic Dictionary:
The inordinate appetite for sexual gratification, one of the seven
deadly or capital vices. God has placed in the organs of generation a sensual pleasure in order to induce man to do what is necessary for the preservation of the race. To seek this pleasure, or consent to it, outside of matrimony, or contrary to the rules of married life, is siaful because it is contrary to the order established by God. Even to lust with the eyes, or heart, is already a sin, as Our Lord teaches in the sermon on the Mount.**
For example, I don’t think that it’d be sinful for me (a man) to
say to myself “that woman looks beautiful” because all I’m doing is observing the fact that God designed her with a natural feminine beauty, that as a man, I’m naturaly attracted to. However, where lust (and therefore sin) comes into play is if I where to have sexualy explicit thoughts about her. Observation that, through temptation, turns into lust is sinful.
I don’t think you having a crush on someone is realy all that bad
(though if you wher an older woman I’d say that it’s a sign of
immaturity). The fact that you find men attractive is natural because you’re a woman. I think that renting a movie just because the actors are good looking isn’t necessarily lustful, but just more of a superficial reason for renting a movie (as opposed to renting a movie because it has a meaningful plot and well developed characters (aka - having substance)).
I also don’t think that letting you’re concience be you’re guide
is a good idea (especialy in this day and age). The reason being is that some people’s conciences are not well formed and informed, while others are to the point of scrupulosity. I think that mentality is what can lead people into dissent from church teaching.
If you think you are struggling with issues of lust, pray the Rosary
as often and as much as you are able to. Our Blessed Mother is the Queen of purity and so she would be a very powerful intercessor. Also try praying the Divine Mercy chaplet, fasting and penance, all of which are very powerful weapons in our daily spiritual battle against satan and his temptations.