So - you told him it could do irreparable harm to the relationship and he chose to go anyway. If there are no consequences to his actions, why would he stop? I mean, if he can have you at home, taking his phone calls, waiting patiently - AND eat his cake too - why not?
He is in such a fog, it hasn't occurred to him that he could actually lose you over this. And he may need a dose of reality before this is all over.
What are you going to do when he comes home? What will you do if he says he's "confused" or that he wants to continue a relationship with her?
Not everybody would handle it the way I would, but if it was me? I'd stop taking his calls while he's there with her. I'd let him take a cab home from the airport. And I'd have his stuff packed and waiting for him on the front porch with a nice note about how I deserve better treatment and here's his reservation at the local motel.
(And, if you haven't done it already, check all the financial accounts and consider removing half of the savings to a separate account if you can - he's a "flight risk", and sometimes guys in this kind of fog empty the bank accounts to run off with their dream girl.)
Coming home to a nice cooked meal and his cozy bed is one thing. Coming home to be slapped in the face with the consequences of his actions is another. It's not the right choice for everyone, but you do need to have some healthy boundaries - for me, this would have far exceeded mine.
