I am in deeply in love with a chaste Christian guy and i am a Muslim 20 year old girl. We want to get married. So does Islam allow me to marry him?
Marriage of a woman with a non-Muslim man, whether be it from ahl al kitab or not, is not allowed. This article gives the actual answer, but I will quote the core reason here:
Even if we deny Jesus being the son of God, we give him as much respect as a prophet as we give to Moses and Muhammad. So, if a (Jewish or ) Christian girl marries a Muslim man, she can be confident that her religious views/Holy men will not be dishonored. But, if a Muslim girl marries a non-Muslim, it is certain that he whom she regards as God's Messenger will be abused/denied by her husband's family, and perhaps even by her own children.
Although you mentioned that
I am in deeply in love with a chaste Christian guy
But being deeply in love in some Christian guy (even he is chaste Christian), is not considered as an Islamic reason for getting married with him.
I also saw that you pointed in a comment that:
May i know why? If a muslim man is allowed to marry a non-muslim then why not a muslim woman have the same right?
The reply (as a reason) could be that the creation of women is delicate and seemingly due to that reason they (women) would get more effect from their parties (husbands) and perhaps it could be leaded to change their religion after a while.
To be honest I found the mentioned matter in a site, but I myself had such a view regarding this matter too. Since commonly men could have stronger effect on women, on the other hand the religion could be considered as the most issue of the life, then logically it is not worth of while to miss our saint religion because of the love in person which it is not clear what would be its result in the future.
Since the Quran permits 'everyone to his faith' there is no reason why you can't marry a Christian guy. However, consider being accepted or welcomed in each others families. Also consider the issue of children and how they will be raised. While love between you and him should be respected, consider the above issues that can make your life easy or difficult.
May Allah be with you.