You had the opportunity to do an honorable thing.  Now, as far as this boyfriend is concerned, that opportunity is gone forever.  It was entirely possible for the two of you to get back together again sometime in the future, partly because he would have had good memories of you when you parted.  That's all gone now.  When you tell him what went on, all your wonderful memories will go up in flames and all he'll remember is that you're a cheater.  Not likely he'll want to get back together with you on that count.  You could try to withhold from your soon-to-be-ex-boyfriend, but that would make you a liar as well as a cheater.  Not something you would want to base a relationship on, would you?  And your "friend" is always out there ready to tell his tale.  Garth Brookes wrote a song in 1995 called "Unaswered Prayers".  Tell your STBX to listen to it sometime.  If fits quite well with your situation.

Why were you at a friends flat drinking when you already have a boyfriend?  Is it because you wanted the sex then?  You are right your boyfriend does not deserve you.  Love is not about sex.  Love is not a feeling.   Love brings about wonderful feelings, but it isn't the feeling itself.  It is a choice to be focused on one person and make a commitment to them.  Otherwise it is just cheap lust.  Learn how to love, multiple sex parters won't get you there. 

OK.  You have one major issue -- the "need" to have sex with multiple people.  While it is true that most of us (me included) have had sex with multiple people, it certainly wasn't out of a need for multiple partners to feel fulfilled.  It was more like this:  (1) eventually you end up having sex with someone, (2) typically, that relationship breaks up for reasons that have little to do with the sex (3) you eventually meet someone else, there's a vibe and ultimately you end up having sex with that person.  The pattern repeats until you find your Mr./Miss Right, and get married and live happily ever after.  IF (I did say if) you are one of the lucky souls who finds a permanent life-long relationship with the first person you ever had sex with, I say you are blessed indeed.

Beak up with the boyfriend as soon as possible.  But tell him of your experience with your "friend" before he does.  If you don't the situation will only get worse.  You are in your early 20's so it is only natural to experience life more.   Don't worry about your "friend" being mad, this is your life not his and who really cares about his moods.  He tried to coerce into sex.   So this is a bit of advise: go out into the world, travel and meet people, experience, life & sex, go overseas or take a car and just drive to where ever you want to go to.  Life is too short to be wasted on people who put in second place and basically use you for their on gratification.  Put yourself first, in a year or so you'll look back with all this in a different perspective.