I think she probably had someone cheat on her in the past maybe and this is bringing up past hurt eventhough she probably knows deep inside it was just an accident. Somehow you need to let her know that you are not like anyone in her past and that you love her and have no need what so ever to date other people or test those waters. This was an accident. Also suggest that the both of you will only drink so many drinks from now on. Never again get sooo wasted when you are with other people. This will hopefully calm her fears. Maybe even go as far a signing a contract for your love and commitment to each other and the fact that this would never happen again. If all else fails I would suggest you go see a counselor together. Show how commited you are. I think its important to listen to her and ask her what she needs before you start talking and trying to reasure her. Just repeat to her what she says so she knows you understand her ottally. Good Luck!!!