Want to know if my partner is faithful.?

Mine is a weird love story. I met a girl who is depressed due to her love relation with a guy who has many girl friends which worried her a lot. I liked her from beginning and pitied her position. She became very friendly to me in no time.She too made many friends using social networking to teach him a lesson. She got fascinated to my truthfulness and caring. She once asked me suddenly that she wants to divert from him and want to couple dance with me. I never had such feelings for her except as a friend. Looking at her condition I accepted so that she forget him and be happy. She then said that the dance involves kissing too. I was shocked. I did kissing and all with her. We soon had physical relation too. Now she wants to marry me. I also want to marry her because I know the value of a heart. Now my mind is not accepting this change at all. I am worried that I am going to marry a girl who has a relation for an year with someone else. She said that she didn't have any physical relation with him at all. But I still doubt that can there be lovers who didn't have any physical relation staying with each other for an year, that too she is very flirtatious. We three once attended a gathering where we had to sleep together. At that time they slept beside each other for 3 days when I was beside him only. What worries if they didn't have any physical relation how can they sleep beside each other so easily. Don't think am a prude but my mind is not accepting this. I want exact clarification. She says that am a perfect man and she thought a lot before committing with me. Will she be faithful with me? She is a gemini and am a tauran. Her lover is libra. She is much concerned about her beauty and attraction. She believes anyone easily and talk to any new person as if she knows them from years. She always enters third person in conversation which makes me piss off. Will our relation last. Am a stable man who is centered only to my lover. She is fond of relating to many people around her. She is not so deep in feelings for me as I have for her. If I say that I don't like her being with male friends and such things she gets immediate turn off on me. Finally what I want to clarify is there any possibility that she had any physical relations before to me. Can she stand on relation with me for life time with that behaviour. These questions are eating my brain daily. Please let me know what to do. Can she show the love she showed for her first love. Tell me please.