patalikopoi,
You are just assuming that your wife had physical affair with her bf and you are not sure about that. She could have lost her virginity while playing with machines. As long as she is committed now, I dont think you have to leave her , like the facebook user has posted below  .
Lets say you found out she had sex with her ex boyfren and you divorce her.  Have you think about what  will she  do afte that?
If she get married with some one after that,  you probably will be ashamed coz some one marries her even after knowing that she was divorced ( forget abt sex). If not, she will be devastated coz maarying again in Nepali culture is still rare and you may find yourself guilty later. 
If I were you , i wouldnot  try to raise the past issue. just dont let her do anythin stupid now.  There is one saying in nepali but I dont know exactly how it goes , but the summary is "if you are too curious, you will find terribly wrong thinng that you are not supposed to see/find". 
I understand the situation you are in is very complicated. You do not want to stay with her, but the other things around you do not let you leave her. We, as a sajha user are just giving you our opinion of what we'd have done if we were  in your place but the family/culture one was grown  makes a big difference in making decision. So analyze all the inputs from here, but dont make your decision totally based on the sajhali's input. This would be one of the very important decision of your life and if you make mistake , you can not do anything other then to repent on that. You cannot blame sajha users on that , and even if you do , that will not matter us or make any difference.
Think carefully and  make wise decision so that you dont have to regret again on your decision.