Is a hand job a sin if you love her and it most definite does not lead to sex?
A "hand job" is slang for mutual masturbation; using the hand to stimulate the genitals. Saying that it does not lead to sex is a misnomer. It would be more accurate to state that in this case intercourse did not take place, but in a sexual relationship the touching of the each other's genitals is a part of foreplay that typically leads up to intercourse. In other words it is a part of sex, you just didn't complete it at this time.
Paul stated, "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). Paul is not talking about holding hands. He is stating that men and women ought not to sexually touch each other outside of marriage. Sex and the things that lead up to sexual intercourse only belong within a marriage.
The problem with sexual touching is that it naturally leads up to sexual intercourse. Though not your intention, people don't think well when they are sexually aroused. To claim it won't happen to you is foolishness. It is a part of the deceptiveness of sin. You think that because nothing more than a little sexual pleasure occurred, then it is not so bad. But eventually the novelty wears off so you try a few other things which aren't that much different. Eventually you get to the point where you ejaculate, but you argue that since you didn't have your penis in her it isn't so bad. Gradually you go further and you're arguing that so long as you don't actually ejaculate in her, it isn't so bad having your penis in her for just a short while. Step by step your resistance to sin is worn down. That is why Solomon said concerning adultery, "Can a man take fire to his bosom, and his clothes not be burned? Can one walk on hot coals, and his feet not be seared? So is he who goes in to his neighbor's wife; whoever touches her shall not be innocent" (Proverbs 6:27-29). Even though it just started with touching, Solomon said a man cannot pretend innocence. Just as you know that hugging a burning coal is going to burn you, everyone knows that sexual touching is going to eventually lead to sex. And when it does happen, your argument that "I didn't mean to" is simple foolishness.
Yet, there is another thing to consider. While giving or receiving a hand job, I can say for certainty that you weren't thinking about tomorrow's homework assignment. "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 you didn't commit actual fornication, because you were thinking about it and your desire for it existed, the only thing between you and the actual sin was your opportunity to get your penis in where it didn't belong. Because you strongly desired (lusted) for fornication, you were just as guilty of that sin as another person who actually committed fornication. "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).
But you also attempt to excuse your sin because you "love" her. My response is that you really don't love her. You are lusting for her body, but that isn't love. "Love suffers long and is kind; love does not envy; love does not parade itself, is not puffed up; does not behave rudely, does not seek its own, is not provoked, thinks no evil; does not rejoice in iniquity, but rejoices in the truth; bears all things, believes all things, hopes all things, endures all things. Love never fails" (I Corinthians 13:4-8). Notice the three phrases that I highlighted. Is fondling another person's genitals something done in polite company? Does it cause a person to think pure thoughts? Is it encouraging a person into sin? Love causes a person to protect the one he loves from all harm. Instead you are encouraging sinful thoughts and sinful actions. You are putting yourself and the one you claim to love in danger of the wrath of God. How is that love?
People often seek out alternative ways to have sex, such as hand jobs or oral sex, in an attempt to have sexual gratification without the risk of sexually transmitted diseases or pregnancy. The problem is that diseases can still be transferred through the skin-to-skin contact of a hand job. True, the diseases will not be spread as easily, but it is not a preventative method. Nor, surprisingly, is it a guarantee that pregnancy will not occur. If you ejaculate, then there is always a possibility that some of your semen might be moved to her vagina either accidentally or purposely. Sperm function regardless of the method used to get it to the appropriate place.
This is probably far more than you wanted to hear, but I hope you see that you and your girlfriend are sinning and you need to take corrective measures to get back on track with God and each other.