Hey, I have a question about sex: I recently turned 18 and I have a girlfriend. I am still a virgin and I'd like to keep myself for marriage, but in the corner of my mind I kind of lust for her when I think about her. I have a feeling that, given the chance, I might go through with it. One day, as a joke, I asked her if she would want to do it and she said "if I wanted to." Now my question is: Is it considerd adultery even if I lust for my girl and never have sex?
Let's start with the extreme and then work our way back.
Having sex without marriage is the word porneia in the original Greek of the New Testament. It is translated as fornication, but many of the newer translations use "sexual immorality" or just plain "immorality" because the translators think "fornication" is too old of a word and not often used. Unfortunately, "sexual immorality" and "immorality" are too vague. They don't capture the meaning of the word well.
Adultery is sex between a couple who are not married to each other, but at least one of them is married to someone else. Therefore, adultery is actually a subcategory of fornication, but sometimes you will see the two words listed together. In such cases, fornication takes on a more narrow meaning where neither of ones having sex are married.
For a list of sexual terms in the Bible and their meaning, see: Sex.
Now that we understand the meaning of the term, we can see whether it is bad. In this, we'll let God answer:
"Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10).
"Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
The word "bed" in Hebrews 13:4 is translating the Greek word koite. It literally means "bed" and it is were we get our English word "cot" from, but the Greeks used the word the same we say "Jack was sleeping was Jane." You know that Jack and Jane were having sex. By the way, we get our word "coitus" from this same word. "Coitus" is the act of intercourse.
Now this should lead to the question of why God said it is wrong. God doesn't make arbitrary rules. The laws He gives are there to make life better. Therefore, there are things about having sex without marriage that makes life bad. For an answer see:
One of the problems when sex enters a relationship is that it dominates it. Everything revolves around sex and the relationship stops developing. Ultimately the relationship falls apart because sex is not enough to hold it together.
If you understand that you may not have intercourse before marriage, the next question is what about the things done prior to intercourse. Can you do anything so long as your penis doesn't enter her vagina?
The first problem is restraint. Most young people discount too heavily the strength of their sexual instinct. This is why I constantly get notes from people saying, "I didn't mean for it to go this far," or "I don't know what happened," or "It was an accident." Such aren't lame excuses. They are the responses of someone who didn't have a healthy respect for the strength of his sexual instinct.
Solomon points out the problem when he asked, "Can a man take fire to his bosom, and his clothes not be burned?" (Proverbs 6:27). You can show a hot coal all the affection you want. You can cuddle it and dote on it and it will still burn you. Your kindness to it doesn't change its nature. How often do you hear someone say, "But I love her!" Solomon's point is that your feelings toward your girlfriend won't change the fact that both of you have built-in desires and capabilities for sex. Trigger them and they follow the instincts built into you.
Solomon also asked, "Can one walk on hot coals, and his feet not be seared?" (Proverbs 6:28). Using the same example of a hot coal, if you walk on it, it will burn you. You can apologize and say you didn't mean to step on it, but you'll still be hurt because your intentions doesn't change what it is. Thus, the excuse, "But I didn't mean for it to go this far!" becomes an empty one because your intentions doesn't change your body's drive.
That is why Solomon concludes, "So is he who goes in to his neighbor's wife; whoever touches her shall not be innocent" (Proverbs 6:29). Though he is talking directly about adultery, the same point is true about fornication. When you start stirring up sexual feelings, you are never innocent when thing go further than you wanted.
That is why we are told not to make room for lust and lewdness. "Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14). Lust is those thoughts and desires you keep battling about taking things even further. Lust is a strong desire for something that is wrong. It is so strong that you start thinking of ways to justify the sin in your mind. Lewdness is engaging in sexual foreplay that gets the body ready for intercourse. The Christian must recognize the danger and not start a sequence of events that can't be legitimately completed.
Personal rules are needed so that you don't start down a sinful path. No touching each other's private areas. No stroking skin to get you or her sexually aroused. No long passionate kisses that leave you out of your mind. You have to treat each other with respect and not as sexual objects. "Now concerning the things of which you wrote to me: It is good for a man not to touch a woman" (I Corinthians 7:1).
With that comes not talking dirty or showing nude or semi-nude pictures to each other. "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). You don't stay pure by sexually arousing the other person.
Now, let's address your actual question. Jesus said, "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). Even though Jesus is talking directly about adultery, it is understood that the same principles would apply to fornication. Jesus is not saying that lust is adultery. His point is that lust for adultery is just as bad as actually committing the act. He is countering the idea that "I haven't touched her, so I haven't sin!"
Therefore, if you daydream about putting your penis into your girlfriend and telling yourself that it wouldn't be so bad because she is agreeable, then you are involved in lust for fornication. While you aren't guilty of fornication, it doesn't mean you are innocent of sin. You would be guilty of lust. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:21-23). The reason God is so hard on lust is because when you justify sin in your mind, you won't stop yourself when you are faced with the actual opportunity to sin -- that, and you are giving approval to sin, even if it is only in your mind. The fact that you said, "I might go through with it" tells me that Satan has got you in the middle of a trap.
The "joke" really wasn't a joke, was it. You presented it as a joke so you would have a escape if she got offended by the suggestion. Sin is not something you joke about. Now that you realize that she lacks moral restraint that a Christian is supposed to have, you are seriously contemplating pulling down your pants. You need to realize that sin is waiting at your door. It is your job to protect your girlfriend's honor and that includes protecting her from yourself.