I believe if you tell her you don't want to have sex before marriage and she still tries to force you she may not truly love you. If you love some one you will do anything you can for them and not having sex until you are married is a very simple thing.
Physcial relationship such as kiss, holding hands, hug, sex is part of love. It's natural that showing one's passion or love through body language. However, if you do not feel ready for this do not go for it. Its not a thing that you feel forced to do. And another thing is I do not know how old you or your girlfriend are, but remember only do things when you feel you can handle it or be responsible for it.