see, my guess is that arnold was just so casual about it.
probably drinking a little high end scotch in his den one night, annoyed at the wife, and the maid walks by. He always flirts, and he knows he can have her if he wants it, she's made eye contact longer than needed.
So he goes for it. she loves it, getting rocked by arnold 15 years ago, movie star millionaire. Arnold's just enjoying the thrill of an in-house BJ or hole anytime he wants.
She probably told him she's on the pill. Once she was preggo, what could he do? Just pay her off and shrug it off. As the kid grew up, his face and frame (and proximity of maid) start to become obvious to everyone.