Well, why do Christians value chastity and purity?
We believe that our bodies are temples of the Holy Spirit. If you don’t believe in a Holy Spirit, or that your body is anything besides a physical thing that has its urges like any other creature— why go through the effort to subdue or discipline those urges?
We believe that our bodies are not our own, so that’s why we refrain from doing whatever we feel like— even when we really feel like it. But if you believe that your body is very much your own, and you can do whatever you feel like with it, as long as it doesn’t land you in jail— that’s what happens.
We believe in chastity according to your state of life. So there’s married chastity and there’s unmarried chastity. But you can run into difficulties with this even between a Catholic and Protestant worldview.
The Bible frequently makes reference to fornication-- sex outside of marriage. St. Paul says that fornicators won’t inherit the kingdom of Heaven— but how many of us who believe in a Kingdom of Heaven ignore that bit, let alone people who don’t believe in a Kingdom of Heaven?