I am going to come back to reading what others have said, I tend to end up replying to their comments and forgetting my own.

Ok, people do things to mess up their relationships at different levels bu this for me, is one of the worst. I know you said he wasn't making excuses, and that's all fine and good but what about the out right deceit? Someone who "ends up" in a bad situation without thinking (still pretty much unexcuseable) doesn't think straight enough to actually form a lie and try to fool their partner into thinking they are with other people. this is thought out, and he knew what he was doing at the time, at the very best one could claim they were too drunk to know what they were doing, but the fact that he texted you about the fictitious situation says that he was coherent enough to make that decision.

For me, this is something I'd have a very hard time getting over. On the one hand I understand wanting to forgive but on the other, if he "gets away with it" and you forgive too easily, that's a sure fire way to make it easier next time for him to think 'well she'll just forgive me anyway" Honestly I don't know the solution and I don't think it's that cut and dry. Even if you tried to forgive, your trust has been broken badly and that's a very hard thing to repair after such a betrayal.