Don't take your own sexuality for granted. Far too often, women demonize us for "only being interested in sex". For one, that just isn't true. We like sports, too.

Second, that's just the way men are wired. They claim to despise us for it, but we're all like that and they keep coming back. In reality, it's what makes us men and draws them to us. They may not like it, but it's biology and it can't be denied.
I'm assuming you are a teenager. If that is so, then two things are true: 1) you are most likely
not going to marry this girl, and 2) you are nearing your sexual prime. Your sexual prime is the point in your life when you get to experiment the most with sex, different partners, and really come into your own, sexually. As an adult of 34 years, I recommend that you don't disqualify the value of sexual experimentation. What you do now will determine your sexual behavior for years to come. If you repress your sexuality now, you will have a difficult time trying to reawaken it when you are an adult. If you open yourself up to your sexuality now, you will continue to be open and have more experience and partners as a result. Of course I recommend that you do so responsibly.
While my advice is not right for everyone, I'm just telling you what I wish I knew then. I didn't know that I
could be open to my sexuality. I thought it was my job to be ashamed of my sexuality, because that's what women told me. They were wrong and/or pushing their own believes on me. Your sexuality is nothing to be ashamed of. If you are a teenage male, you have
every right to be a raging ball of hormones. That's how God made you.
It may be that you still want to persue this relationship. It's very probable, as described before, that she will eventually subcumb to her own biology and give in. However, there is no reason you should apologize for desiring sex. "You're only interested in sex" is
not an insult. Breaking up with a women because she won't put out does not make you a pig, it makes you a guy who knows what he wants. Like I said, there's almost no chance you're going to marry her anyways. If you did, it would almost certainly fail.
Reframe your mind, and it might just change your persepective. Be aware of the changes that are happening to you. They empower you, and are nothing you should ever be ashamed of.
BTW: I am in no way suggesting that you pressure this girl for sex. Never do that.