Even though you'd only slept together once, your girlfriend felt sorry enough for you to agree to sleep with your boss to save your job. It was a little jarring to hear her say she loves him after, since she hadn't even said that to you yet
At parties your boss always boasted that he could make any woman spread her legs for him. In a drunken state you bet, for a substantial raise, that it wouldn't work on your girlfriend. It took him one hour, and she didn't let him pull out.
Whenever you are fired your girlfriend goes to get your job back so your boss fires you every week just so he can fuck her face and make her swallow his load.