As its been said, sin is sin.
We should all make efforts not to sin. Does it feel wrong to look at inappropriate content? Sure it does. Does it feel wrong to be sexually attracted to a girl you know in school (or whatever) of the same age? No.
fragmentsofdreams pointed out the difference between lust and sexual desire perfectly. If I see a girl, and only want to know her just to have sex, or all I want to do with her is have sex, that is lust. Instead, we should want to know that person, to long to have a relationship with them. Get married, have kids.
And despite all this, don't worry yourself to death over sin. As God said:
"I, even I, am he who blots out
your transgressions, for my own sake,
and remembers your sins no more." Isaiah 43:25
As you can see, God doesn't want to remember your sins. Imagine suddenly remembering every bad thing your best friend or your brother had done to you. You would hate them. God, all knowing, doesn't want to remember your sins for his own sake. He loves us.