Tuger, according to the bible fornication is wrong, that is, having sexual relations outside of marriage.
1 Corinthians 6:18 "Flee from fornication, Every sin that a man doeth is without his body, but he that comitteth fornication sinneth against his own body" And 2 Timothy 2:22 says "Flee also youthful lusts, but follow righteousness, faith, charity, peace, with them that call on the Lord out of a pure heart" (KJV bible)
You need to explain to your girlfriend that sex is best in the context of marriage only. The bible says let every man have his own wife.
Hebrews 13:4 states according to the NIV bible that "Marriage should be honored by all, and the marriage bed be kept pure for God will judge the adulterer and all the sexually immoral.
Another good thing may be for you to ask your minister for scriptures about sex before marriage or a local minister.
Hope I helped you.
Missy