What sexual acts should not be committed in a relationship before marriage?
"Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
From the time that God created the world, His intention was that sex would be freely expressed within marriages between a husband and wife. The word "bed" in Hebrews 13:4 is a translation of the Greek word koite. It literally means "bed," but it also refers to the actions that take place in bed -- in other words, sexual intercourse. The English word "coitus," the actual act of a man joining with a woman in sex, comes from this Greek word. Within marriage sexual intercourse is proper, but outside of marriage, it is a sin.
The word "adultery" refers to sexual relations where one of the two involved is married but not to the current partner. The word "fornicators" is a broader word that includes any sexual activity outside the bond of marriage.
In this world of double talk, people often search for technicalities that allow them to engage in sinful activity because it is not expressly forbidden. Not long ago, then President Clinton tried to excuse receiving oral sex by an intern in his office by declaring that it wasn't really sex. By his definition intercourse did not take place and he was not actively participating, even though he did ejaculate; therefore, it was not sex. Of course, no one is fooled by this playing around with words.
Paul told the Corinthians, "Now concerning the things of which you wrote to me: It is good for a man not to touch a woman. Nevertheless, because of sexual immorality, let each man have his own wife, and let each woman have her own husband" (I Corinthians 7:1-2). Because of the context, we realize that Paul was not talking about accidentally bumping into a woman as you walk down a street. He is using it in the same sense as Proverbs 6:29, "So is he who goes in to his neighbor's wife; whoever touches her shall not be innocent." Both I Corinthians 7:1 and Proverbs 6:29 are referring to sexual foreplay. If fornication is wrong, then the foreplay that leads up to the actual intercourse is equally wrong.
Jesus told his audience, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28). Hence, to entertain thoughts of engaging in sexual acts outside of marriage is equally wrong. Sin starts in the heart. To entertain the idea of sinning is just as wrong as actually engaging in the sin. If nothing else answers your question, this one should cinch it. What sexual acts could you engage in prior to marriage that doesn't fill your mind with ideas of immoral sexual deeds?
Paul told the Thessalonians, "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God; that no one should take advantage of and defraud his brother in this matter, because the Lord is the avenger of all such, as we also forewarned you and testified. For God did not call us to uncleanness, but in holiness" (I Thessalonians 4:3-7). Christians are to keep themselves pure by abstaining from fornication. As an example, Paul said we should not engage in "passion of lust." The Greek words behind this phrase are pathei epithumias. It literally means longing after lustful passions. In other words, Paul is saying it is wrong to stimulate your sexual desires outside of marriage. Looking at pornography would be a prime example of what Paul had in mind when forbidding "passion of lust."
Finally, to the Ephesians Paul wrote, "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God. Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. Therefore do not be partakers with them" (Ephesians 5:3-7). Paul lists three things that Christians need to avoid: fornication (sex prior to marriage), uncleanness (things that would encourage fornication), and covetousness (greedy desire for what is forbidden). By saying "let it not even be named among you," Paul wants a Christians behavior to be such that these things are so foreign to a Christian's life that people would never associate a Christian with these deeds or thoughts. In order to accomplish this, Christians must avoid filthiness (dirty talk -- speech with sexual content or profane words regarding sex), foolish talk (senseless chit-chat -- gossip about sexual exploits), or coarse jesting (witty remarks regarding coarse things -- discussions that include sexual innuendos or jokes with sexual punch lines). You cannot talk like a person who thinks nothing of fornication and then expect people not to think you do so when no one is looking.