Is oral sex between a married man and woman a sin and if so where can it be found in the Bible?
The act of sex is to be restricted to married couples. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Such acts of sex are not just those related to vaginal intercourse. In speaking of the sexual relationship between a husband and wife, the terms used are broader. For instance, Paul stated, "Let the husband render to his wife the affection due her, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does" (I Corinthians 7:3-4). Paul is not saying that a person can do whatever they want to their spouse. The word rendered "affection" means kindness or goodwill or secondarily a gift of kindness. If, as a part of a sexual relationship, the husband or wife enjoys oral aspects of sex, then it can be affectionately shown. However, if the spouse finds such relations repulsive, then it would not be kind to press them into doing such acts. If it is demanded, then it becomes a selfish act for one's own sexual satisfaction and no longer a loving gift given to the other.
"Let your fountain be blessed, and rejoice with the wife of your youth. As a loving deer and a graceful doe, let her breasts satisfy you at all times; and always be enraptured with her love" (Proverbs 5:18-19). In case it is not clear, "your fountain" in this passage is referring to the husband's genitals. Notice that the act of sexual enjoyment is not restricted to only intercourse.
"Make haste, my beloved, and be like a gazelle or a young stag on the mountains of spices" (Song of Solomon 8:14). "Mountains of spices" in this verse is referring to the wife's breasts.