If I picked A it would be like it had never happened in the first place, except it did. But as far as I am concerned, I wouldn't remember it so I'm never going to know the difference.
If I picked B, everyone would annoy me. For the most part, I don't care what people think, but I am taking 'everyone" to include my mother, best friends, boyfriend, and his family. So effectively I'm choosing to be in a situation where I cheated...and by "swears you did", you actually mean that their opinions cannot be swayed.
So my options are essentially to have sex, or to "have sex" and then deal with losing trust from/hurting the people most important to me??
Sex is a physical act. It's supposed to be fun. Clearly, it wouldn't be with an ugly person, but neither would the world end for me. I move on. Bonus, I won't remember. And honestly, at the end of the day, my life is going to be more adversely affected by B than A, which is what I'm going to pick.
Hypothetical questions don't usually make me pick the option that I might find to be less "morally superior", so well done for that...You should ask more of these