You guys might not agree with me, but here goes...
Why do you need to tell your bf? B.c the guilt is eating you up, or he 'deserves' to know you betrayed him? Have you and this friend of yours agreed that it was a mistake never to be repeated? If so, why do you want to hurt your bf and his gf with the truth? So you can sleep soundly? Or will it benefit them somehow? I'm not trying to be mean, and I apologize if I come across as a total biatch, but what is the payoff here exactly?
If it were me in your shoe's i'd spare my bf and the other gf the knowledge of my betrayal, b.c, really, how is this helpful to either of them?
Now... If you had sex with someone else, THAT'S different, b.c there a things such as std's and unplanned pregnancies. THAT, I would share, b.c my bf could unknowingly be at risk. I know everyone preaches that honesty's the best policy, but in this case I ask, "for whom"...
Just my 2c.
If you do confess, I sincerely hope it works out, and I agree don't blame the alcohol, own your mistakes.