This is not an easy situation. Failure to have sex no doubt cost me a chance for marriage with the only real girlfriend that I ever had and the same thing was almost certainly true for another guy who was in the same Bible study group that I was in decades ago. Your story is interesting, as my former girlfriend also had a sexual history. I suppose that when such is the case, then it is more likely that one will expect sex before marriage.
For some strange reason, I have always remembered a sermon at a Baptist Church that I was visiting, way back in the 1960's. The pastor asked us, if it turns out that the Bible is wrong and that the Jesus story is not real, what have we lost by living for God? Well, in terms of our discussion here, one might not have lost much back in the 1960's, but my Bible study friend and I both lost our girlfriends in the 1980's by refusing to turn away from God's standards. No doubt the world of the 2010's is more like the 1980's than the 1960's and even within many Christian couples today, there seems to be an expectation for sex before marriage. Now I am not trying to convince you to give in to your girlfriend's request. However, I believe that it is important that you realize what could happen. At some point, she could well feel that you do not love her if fail to show her such in a physical way. Still, if she really loves you deeply, she should be willing to respect your religious convictions.
I do not regret my decision back in the 1980's, mostly because I do not believe that marriage would have worked out in the end with my ex-girlfriend. As for the subject at hand, I see both sides. However, I guess I am very old fashioned. If a couple has sex before marriage, then what is left for marriage? You have already done it and probably many times with most couples. I guess I believe that waiting for marriage says something special about a couple. It says that they really trust each other enough to believe that their love will succeed in marriage, even though they have yet experienced the ultimate expression of their love.
I cannot tell you what to do. This is a deeply personal decision. You know what God's standard is, so no need for me to be preachy. One more thing to ponder is that pregnancy is always a possibility if you have sex now, even with protection. If she does get pregnant and things do not work out, then you will have a child to support and no marriage. Also, there is the possibility of sexual transmitted disease, since she has a sexual history. I believe that in many/most states, couples are required to get a blood test before they get married. However, you cannot ask her to do such now though, as that would surely end the relationship. I will say a prayer for you tonight. God bless you.