Hey Mattheus09,
Yes they are normal because we are sexual beings with a drive that is their for a purpose, but we are also fallen beings and so can turn to disordered desires. When do impure thoughts become sins? When you start desiring them and manipulating them because you want to feel some sort of physical or emotional pleasure from them. If you do it briefly, then it would be a venial sin and if for a longer period of time (like really fantasizing them out) then it would be more mortal (as long as it fits the three criteria of mortal sins). Your case sounds like a normal person who has temptations (even St. Paul had them, “thorn in the flesh as he called them.”) and you aren’t actively trying to fantasize about them for pleasure. What helps me is having a uick form of prayer on hand that is short; like “Jesus, save me from lustful desires.” Then i just pray seperately on my own for purity of heart when not experiencing temptation. frequenting the sacraments, holy books, reading the bible, the rosary and others are all good things too. Also, avoid a lot of idle time doing nothing. Then the devil can enter easily because the “line” (as in your mind) isn’t busy with better pursuits. Hope that helps.
In Christ,
Tom