I have taken a vow, and I have remained celibate for a year and a half now. I have remained resolved to be abstinent until I am married, even though I am constantly presented with several opportunities to fornicate. However, one thing that helps me out is masturbation. When I masturbate, sometime my girl likes for me to ejaculate on her. and I like it as well. She doesn't touch my genitals in anyway, but she just says that she likes the warm feeling of semen. Is this a wrong?
There are times when I wonder what happened to morality and common sense. "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" (Roman 13:13-14). You say you made a vow to be celibate until marriage, but you then seek out ways to get as close to sex without crossing what you see as "the line." In your case, the line is not allowing a girl to touch your genitals.
But sin doesn't just take place when you put your penis into a girl's vagina. There is the problem of lust. "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). Sin doesn't start with the action. It begins with the thoughts of doing sin. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, ..." (Mark 7:21). I strongly doubt your thoughts are pure and wholesome while you are masturbating in front of your girlfriend. And there is the question of what kind of thoughts you are encouraging in your girlfriend.
There is also the problem of modesty. You are exposing your genitals for someone, who is not married to you, to see. Since the time of Adam and Eve, men realize that exposing their nakedness is wrong. It ought to be a matter of shame. "Your nakedness shall be uncovered, yes, your shame will be seen" (Isaiah 47:3). Instead, you've lost the ability to blush, much like the ancient Israelites. "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).
Finally, you are taking a risk. Your semen contains about a half billion sperm. I don't know what state of undress your girlfriend is in or what other things are going on, but all that is needed to cause pregnancy is a bit of your semen to reach her vagina. Sperm don't care if they are put there by a penis, fall there, or place there by a finger. Sperm have tails which allow them to swim to their destination. While ejaculation into a vagina puts them closer to their destination, giving a higher rate of survival, longer distances doesn't make the odds of reaching the egg zero.
Being a Christian doesn't mean just avoiding what you might consider the graver sins. God sees all sins as basically equal. "For He who said, "Do not commit adultery," also said, "Do not murder." Now if you do not commit adultery, but you do murder, you have become a transgressor of the law" (James 2:11). Sure, you've managed not to commit fornication in the last year, but it doesn't mean you are living righteously. Nor doesn't your past avoidance mean you are going to be able to continue staying away from this particular sin when you continually put yourself close to fornicating. Let's just say you do give in one day, would you be able to honestly say that you are innocent given your past history?
It is past time to start living righteously. "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. For you were once darkness, but now you are light in the Lord. Walk as children of light" (Ephesians 5:5-8). You've avoided fornication, but you are still living a sexually unclean life.