I would be very careful before telling your bf if it was me. Firstly,you genuinely don't know what happened, your friend was also very drunk. I would also be cautious about thinking it was rape. Being blacked out doesn't mean you were unconscious, unwilling and unable to give consent. It means you don't remember. You may have been willing or even initiated it. Or you may not. you just don't know. I don't think it's a good idea to try to persuade yourself or him that you didn't give consent in the hope he understands. I'm female and have been in just the same situations when younger so do understand. However, your bf may wonder why you were in a house with men,drinking etc and given his views on fidelity,do you think he will forgive you? Maybe use this situation as a wake up call to stop drinking .It need never happen again and you can move forward. Yes, you've made a mistake, we all have. I don't think you need to ruin your relationship and the rest of your life over it. The shame and memories will fade. Drinking will only get worse -please get help