Is it a sin to masturbate with my girlfriend? By masturbating with my girlfriend, I mean that we do not touch each other's genital; but we masturbate together as a couple.
The general rule since the fall of man is that a person doesn't expose his genitals to others. We make efforts to keep our privates private. Of course there will always be times when it can't be avoided, yet we should make efforts to minimize it. Being seen naked in front of others ought to trigger a sense of shame. "Your nakedness shall be uncovered, yes, your shame will be seen" (Isaiah 47:3).
Let me use the example of Ham (Genesis 9:20-27). Ham's father, Noah, sinned by making wine and getting drunk. Ham accidentally walked in on Noah while he laid naked in his tent. Rather than be embarrassed by the situation, Ham went to his brothers. We aren't told what he said, but it is likely he thought it was funny that their father was acting so poorly. I want you to particularly note Japheth and Shem's response: "But Shem and Japheth took a garment, laid it on both their shoulders, and went backward and covered the nakedness of their father. Their faces were turned away, and they did not see their father's nakedness" (Genesis 9:23). I'm sure that over the years there were many occasions when they might have seen their father without clothes. They could have just walked in and covered him up, but they were making a strong point. Their father, because of his drunkenness, was in an embarrassing state, and they refused to see their father lowered in their eyes. Thus they went out of their way to make sure they didn't see Noah uncovered.
The implication in this passage appears that Noah wasn't just unclothed, but also sexually aroused. There are other passages that also connect the term "nakedness" with sexual activity (Leviticus 18:6-19).
One of the complaints God had against ancient Israel was the fact that they lost their sense of shame. ""Were they ashamed when they had committed abomination? No! They were not at all ashamed; nor did they know how to blush. Therefore they shall fall among those who fall; at the time I punish them, they shall be cast down," says the LORD" (Jeremiah 6:15; see also Jeremiah 8:12). It ought to be embarrassing to be naked in front of a girl, let alone masturbating.
It is improper to expose your sexual activity before other people. It should be reserved for you and your wife alone. Here then is where we get to the root of the problem. You are proposing not only to expose your nakedness to girl you are not married to but also do sexual acts in front of her. Guys are visually stimulated, so what you are proposing is to generate lust in yourself to get sexually aroused. Though you may not be touching her, that is not what is in your mind. "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). God specifically forbade doing lustful things to stimulate sexual passion. "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" (I Thessalonians 4:3-5).
Honestly, do you think it is going to stop there? A big element in sin is the excitement of doing something you know you should not be doing. After a while masturbating in front of a girl will be old. And since you are already naked, it would not take much to move matters to more serious forms of sex.
So the reality is that you are merely looking for ways to get as close to sex without going all the way, but you are ignoring the strength of your body's desire for sex. When you get sexually aroused, you don't think things through. You go with the flow of the moment, and since you have already removed most of the barriers to not having intercourse, it won't take much to push you over the edge. Then you'll be writing to me saying, "I don't know what happened. We didn't mean to go that far. Could she be pregnant?" Don't start what you can't lawfully finish.