You have not mentioned that whether you wanted to marry her initially and then taking a back step or you are not interested and she is compelling you through threat. If you were doing a time pass then you have choose a wrong one. She must have developed emotional dependence and in such case it is very difficult for her to face rejection. However, you should not succumb to her threats. One can not agree to marry on such threats. It is a kind of emotional blackmailing and that may run through out the span of your relationship in various forms, once feared.