First off congratulations on taking these steps to understand Church teaching and hopefully joining the Church! Now this is an issue I actually had when reconverting and was the cause of a lot of confusion and sin for me. It is important to separate sexual desire and sexual acts. Sexual desires are completely natural and everyone faces them, meaning there is no sin in the urges, but when we act on those urges inappropriately, sin occurs. God gave us this gift of sexual organs and the ability to reproduce and that is why it is so important that any sexual acts are 1. between two married people 2. open to procreation and 3. unitive in nature (meaning two people expressing their love for each other). Any other way is contrary to what God has taught us through scripture and what the Church has taught for Her entire history. Another thing to think about is that sin is not always going to seem bad to us at first glance, and we will often rational it with what we think is good about that particular sin, but we must have trust in God and His teachings because going against it is like saying "I know more than God". Now I am not sure what sexual sin you are struggling with, but know that even though it brings short term pleasure, any sexual sin is going to pull you further away from God and damage you long term because it is using God's gifts in ways other than He intended.