It’s completely your right to decide whom you want to marry. As you both were sexually involved with consent and it wasn’t forced then she can’t force you to marry you. Marriages should be done willingly and not being pressured. We would suggest you to have a good communication with her. You should try to change her mind. She is wrong here, she can’t forcefully marry you because you both got sexually involved. This is not the right way of getting married or forcing someone to get married.