First of all, if you are agonizing about whether or not to tell your boyfriend, perhaps it is not wise to do so on an otherwise silent - and packed - train carriage. Secondly, if you are agonizing about whether or not to tell your boyfriend, perhaps it is not wise to phone up your entire circle of acquaintances and tell them about it so that everyone knows about it except him. It will not stay that way for long. Thirdly, from what I gather, having been seated in the privileged position directly in front of you, you got drunk with a male friend, who also has a girlfriend. You let him sleep at yours, during the course of which he made a pass at you. You didn't shag him. You didn't even kiss him. Even if you are currently lying to your friends and yourself, this is not the end of the goddamn world. Why don't you face up to the fact that what you're really after is the pleasure of making a tearful, heartrending confession? And the romantic, painful reconciliation scene? I can almost guarantee, however, that the way you're going about it ensures that should you eventually decide to tell him, this is not what you'll get.
PS Explain to me how it is that you did not spontaneously combust from all the hatred being directed at you by my fellow commuters.
To Christina's boyfriend,
Maybe she's not a cheater, but she definitely is a drama queen. This will happen again. So unless you happen to be equally fond of public scenes of remonstrance and floods of tears, I'd suggest a quiet exit at your earliest convenience. HA! Ahem, excuse me, something in my throat.
Good luck with that,