I have a girlfriend. We are both 14 years of age and are both Christians. We have never touched each other in sexual ways. We know that is to wait until marriage. Is it a sin to look at my girlfriend's breasts? Is it okay to look at hers and only hers?
I assume that you mean to look at your girlfriend's exposed breasts. Since you qualified your question that you would only look at your girlfriend's breasts, you are telling me that you realize there is something wrong with looking at women's breasts in general.
We keep our sexual areas covered. "And those members of the body which we think to be less honorable, on these we bestow greater honor; and our unpresentable parts have greater modesty" (I Corinthians 12:23). For a woman, the breasts can be covered, but the shape is still there. It is a sexual area that you can't avoid but notice that it exists. It is different from a man and for that reason it is a banner that says "female."
One reason the breasts are attractive is knowing that they are off-limits. "Stolen water is sweet, and bread eaten in secret is pleasant" (Proverbs 9:17). Even ordinary things become exciting when we know it is wrong in some way. Thus, knowing that a woman is to keep her breasts private makes them just that more exciting and arousing.
This then is the core reason looking at women's breasts is wrong: it produces lust within you. It doesn't matter if we are talking about a woman you don't know or your girlfriend. The problem of lust remains. "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). Lust is when you strongly desire something that is not allowed. Looking at your girlfriend's breasts is going to get you sexually stirred up and in that arousal you are going to start imagining yourself doing things that are not proper for a Christian to do when he is not married.
While you are boyfriend and girlfriend, that is not the same as marriage. Intentions regarding the future doesn't make it a reality. Therefore, you can't compromise and claim to do things that are only allowed for a husband and wife and excuse it because you like each other.