"The idea that this woman and kid have no idea that they're living with a robot is just plain ludicrous."
No, it isn't. From their perspective, there's no such thing as a robot covered with real human flesh, and AI that can pass for human. So why would the idea that he's a robot even occur to them?
"They don't notice that he never uses the toilet?"
How do you know he never uses the toilet? He obviously isn't required to, but there's nothing preventing him from going into the bathroom for a minute every few hours in order to present the appearance of being normal.
In the first Terminator movie there was supposed to be a scene showing the Terminator eating, because he'd obviously need to eat in order to sustain his human flesh. So it isn't far fetched that he eats; it would be far fetched if he didn't need to eat.
Again, easily simulated.
"Penis doesn't work?"
First, how do you know? Second, even if it didn't, he stated that he didn't have a physical relationship with that woman (also, it's never established in the movie that he's married to her either).
"Acts like an autistic droid?"
There's no such thing as an "autistic droid." Autism is a human condition, so even if someone thinks he acts autistic, that doesn't in any way suggest that he's a "droid."
"Breaks furniture because he's so heavy?"
It was said that he weighs 400 pounds, which isn't heavy enough to break decent quality furniture just from sitting or lying on it. Andre the Giant weighed over 500 pounds and he could sit down without breaking furniture.
"My brain left the theater at this point, unfortunately my body didn't follow."
The movie sucked hard, but not because the family didn't know that "Carl" was an android. He was designed to be an infiltrator; passing for a human is his specialty.