What does the Church say "no" to? Things that are not good for us:
- sex outside of marriage.
- homosexual sex.
The Church says "no" to the culture's messed up idea of our sexuality, which is ultimately a "yes" to love and sexuality as it is meant to be. It says there "no" to the thought that we are supposed to seek out our true happiness through sexuality that leaves behind love and focuses on pleasure alone. We need the laws of the Church to help us have a proper guide to what is good, true, and beautiful.
It also helps us answer the question - what is the purpose of sex?
Here is Fr. Barron on the thinking behind the Catholic Church's teaching on sexuality.