All i can tell u is my experience, I made the same type mistake ur boyfriend made, My girl friend ,both then 18, forgave me, became my wife at 18 and 27 years later we are still very happily married. It actually kinda woke me up, and i wasn't drunk. Oh and it does sound like that girl knew what she was doin from the start. So search your heart and do what you think is best for you.
When you opened the door and found your boyfriend and the other gal having sex, he did NOT look confusedly at her and say "Oh %&#@!!!! I thought this was YOU!" or "Where am I, and what am I doing?!?" or (looking at the woman he was having sex with) "Who are YOU?" He didn't act confused. He was not passed out, he left the room with her and proceeded to have sex. He was not mistaking who he was with, he wasn't in a blackout because he leaped up and begged your forgiveness and remembered everything the next day. He didn't try to say he didn't know what he was doing, he just acted caught.
Blaming it on alcohol -- alcohol releases inhibitions, but what it does is release who is really there on the inside, not take over their body and make them do something they don't want to do. (What if it had been a guy coming on to him? Are you saying that he would have left the room with him and gotten into a car and had sex because he had had so much to drink?)
In short, please do not discount that he did this totally of his own volition. The gal's advances merely told him she was available. He was not forced in any way, even by the presence of alcohol, to accept the invitation.
I think he has destroyed your trust in fidelity in a partner for a long time, if not forever, and that is a lot to forgive.
If your fear is losing such a (surface) perfect relationship, well, please remember that if you are lovable enough for a boyfriend to be talking marriage, apartment and kids, you are lovable enough for other guys to feel that way, too. You picked a weak brother, it is not your fault, you trusted and were betrayed, but that is his problem and not yours. You have the right to be able to trust, and there are men out there who are perfectly trustworthy and their commitments can be believed.
Cheating is VERY difficult to get over. It's ok to forgive and want to work it out, but you have to realize it's not easy at all to move on from this. What Teko wrote is true and something you really need to think about. Once someone loses trust, it's hard to regain. In my opinion, I would say to cut your losses and move on. He had enough sense to make the decision to chase after you, so he knew exactly what he was doing. He's made up some very lame excuses and isn't taking responsibility for his actions. He absolutely needs to own up to them if you both want any chance at all for this to work out. Good luck with what ever you choose to do.
I think teko summed it all up perfectly in her last post. I couldn't have said it better.
Ok. Some may not agree with "once a cheater, always a cheater", and that's fine. From MY life experiences, it's rung true. My biggest issue with this is that you've only been together 2 years. Marriage is supposed to be about a lifetime together (ideally). If he cheated on you 2 years in, how do you know it won't happen again? If you were married already, had kids, or had been together 5+ years, I could understand working it out. Sometimes I think people can work things out. 2 years is a short time, and you sound like you're pretty young still.
I tried working it out with one guy after cheating. I met him when I was 17, he was 25. We got along really well. Fast forward 3 years and 1 child later, I found out that he was cheating on me with a girl at his work. He begged and pleaded for another chance. Said it would never happen again. I forgave him. He cheated again. We've left it as just friends for a few years now. He cheats on every girl he's with. There was no consequence for him cheating. If there had been, maybe he wouldn't be this way now. If I had known then what I know now, I would have never taken him back.
What you do is your choice, but just think about it. 2 years isn't very long if you're planning on spending a lifetime with him ;)
I think the fallout of this deed is what is going to tell the tale. Lets see what happens when, as a result of all this,when this young lady starts distrusting all other females when they are around him,even friends, or the insecurity it is going to invoke when he starts drinking in or out of her presence. Or watching his every move to see if he is looking at other girls, which he will eventually unless he goes blind. When the resentment of the betrayal starts to outweigh all those feelings of forgiveness. With all the intentions of forgiveness feels more like enabling him to do it again, and living with that thought on a daily basis, because, it worked once? Jealousy, distrust, low self image. Quite the price to pay for one night of drunken behavior. To him? No, to her. HE will also pay a price for all of these emotions and at first it will be feelings that he deserves it, then resentment at having to pay the piper for a long time to come as a result of invoking these emotions with his drunken behavior. Can the relationship stand this, long term, in the dating phase? Time will tell. That is one rollercoaster ride I would not willingly take. IMO