Me And My Girlfriend Had Sex Now She Refuse For Sex, How Can I Convince Her For Sex Before Marriage?

I would say that a logical point for your side of the argument is that you both already lost your virginity to each other, and that is something rare and special today. So basically you both have already had sex, so why not still have it? You guys want to eventually marry, so it's not like you would be having sex with other people, it's the person you want to spend the rest of your life with.
That would be your argument. But, sticking up for your girlfriend, I would say that maybe she is on to something. Purity is a virtue in marriage, and even though you guys have done it once already, she may just want to keep the relationship as pure as possible until you guys do get married. You can't be too mad at her for wanting the best possible thing for you guys.
Good luck, you both need to talk this out and come to an agreement with open minds.

Just because she agreed once doesn't mean she would do it again.  Understand that cultural values are so embedded in a person's psyche she may not even be aware of it.  She may not even tell you about it.  

When two people are in love, they inevitably kiss, hug, and touch.  However, at the back of the mind of women always lurks the doubt of the man's real intention towards her especially if she has marriage in mind.  It is healthy and wise for her to do so. Having said that, maybe you should direct your sex drives for now towards exerting effort to really really show her how much you love her and care for her in a non-sexual way.  

Give it time, respect her.  Maybe she is confused and doesn't know what she wants at the moment.  If you want to be a good boyfriend respect her wishes and don't have sex.  If you truly love her you won't need sex.  Don't worry when she is ready she will let you know.