The Church makes it sacred and evil because the Bible says it is only to be between and man and woman who are married to each other. Case closed. Thats what the Bible says and that is what most churches go by. It specifically says multiple times in the Bible that sex is not to be done before marriage or outside of marriage. It is called fornication.
Show me some scripture. I dare ya'
Gal 5:19-20 19 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, NKJV
Eph 5:3-4 3 But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; NKJV
Col 3:5-6 5 Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness, which is idolatry. NKJV
