Hmm, in this situation I think you both have the same amount of guilt. Think about it. In my opinion there are some several situations and the amount of guilt that goes with it. I didn't read all posts (only page 1), so keep that in mind please. But I believed you said this girl has a BF that is/was away for a long time. Naturaly she has some needs that can't be satisfied at the moment, because her BF is gone and yes, this makes here furnable (or whatever you spell that word).
Situation A: The girl has a BF she practicly sees every day and you know each other well:
A1: She fancy's you, which results in sex: Both the blame, but you should know better.
A2: You fancy her, which results in sex: Her blame, but you are a dick.
A3: You fancy each other, which results in sex: Both the blame, she should end her relation ship.
Situation B: Situation A, but there is alcohol in the game:
B1: You both are drunk and she fancy's you, which results in sex: You both have the same amount of guilt (she only has more explaining).
B2: She's drunk and she fancy's you, which results in sex: You have the guilt, taking advantage of the situation.
B3: You are drunk and she fancy's you, which results in sex: She has the guilt and should end her relationship.
B4: She's drunk and you fancy her, which results in sex: You have the blame here, using the advantage, you are a dick.
B5: You are drunk and you fancy her, which results in sex: Her blame and she should end the relationship.
Situation C: The girl has a BF who's gone for a long time, which gives and advantage.
C1: An occasion results in sex: You are always to blame, you take an advantage of her biological needs.
C2: An occasion results in sex, but you DO remind her of her relationship: Her blame all the way, but still....