Yes it is.  This question is becoming increasingly common as young people are told that “oral sex is not really sex,” and as oral sex is promoted as a safer (no risk of pregnancy, less risk of sexually transmitted diseases*) alternative to sexual intercourse. What does the Bible say? Ephesians 5:3 declares, “But among you there must not be even a hint of sexual immorality, or of any kind of impurity…because these are improper for God’s holy people.” The biblical definition of “immorality” is “any form of sexual contact outside of marriage” (1 Corinthians 7:2). According to Hebrews 13:4, only the “marriage bed” is pure and undefiled. According to the Bible, sex is to be reserved for marriage. Period. So, yes, oral sex is a sin if done before or outside of marriage.  God bless you!!! <3