I know that sex before marriage is a sin, but where is this clearly stated in the Bible? When approached with this question previously, the only verses I could recall refer to "sexual immorality" or "adultery." Where is premarital sex defined as adultery and sexual immorality?
Instead of seeking a Bible verse that explicitly says something like, “Sex between two unmarried people is sin,” you would do better to approach the whole issue as the Bible does. God establishes marriage and then says to those being joined together in marriage that they are to be intimately united and that they also become one flesh (see Genesis 2:24 and Matthew 19:5). Thus two things are linked to marriage: union (the intimate joining of a man and a woman as husband and wife) and becoming one flesh (sexual, physical intimacy too). To become one flesh (have sexual union) aside from this marriage bond is never allowed and never spoken of as God-pleasing. It is, however, clearly identified as wrong, as sin (see 1 Corinthians 6:15-18, for example). Any use of the gift of sex aside from the marriage bond is adultery, whether this is premarital or extramarital. God has graciously provided for the sexual desires of men and women to be satisfied only in marriage. To engage in premarital or extramarital sex, before or outside of marriage, is to sin in God’s sight. That is precisely the point of Hebrews 13:4, a verse often referred to in this kind of discussion. “Marriage” and the marriage bed (the Greek text only has the word “bed”) go together and are to be kept pure. Using the “bed” aside from “marriage” is sin that God will judge.
The counsel given in 1 Corinthians 7:9 makes the same point. If a person has sexual urges and the sex drive (a good gift from God in itself) expresses itself within a person, that person has a God-pleasing remedy identified: to be married and thus obtain the right to be sexually active. Before or outside of marriage, sinful lust is sinful lust. Marriage partners may express the same desires for each other and there is no sin. This kind of “lust” is not sinful. (The Bible word used here is also used for legitimate and proper strong desires, even though the English word usually always carries a negative idea).
I suggest that you sit down with your pastor and seek additional clarifications if you wish more. He may also have resource material in his library that can be helpful to you as you seek to serve others who ask you about these things.