Futurama proposes a different reasoning for the inevitably changing human-robot ratio (I couldn't find a clip of it, so.. use your imagination):

Fry: Well, so what if I love a robot? It's not hurting anybody.

Hermes: My God! He never took middle school hygiene. He never saw the propaganda film.

Farnsworth: It's just lucky I keep a copy in the VCR at all times.

[He presses a button and a film title, I Dated A Robot!, appears on the screen. In the movie a couple sit in a cafe and stare into each other's eyes. A narrator walks into the scene.]

Narrator: [in movie] Ordinary human dating. It's enjoyable and it serves an important purpose. [He turns the table over and a crying baby appears. He turns it back again.] But when a human dates an artificial mate, there is no purpose. Only enjoyment. And that leads to ... tragedy.

[The woman behind him turns into a blank robot and the man downloads a celebrity onto it.]

Billy: [in movie] Neat-o! A Marilyn Monroe-bot!

Monroe-bot: [in movie] Ooh! You're a real dreamboat, (mechanical voice) Billy Everyteen.

Narrator: [in movie] Harmless fun? Let's see what happens next.

[The scene cuts to Billy's bedroom where he kisses the Monroe-bot. His mother walks through the door.]

Billy's Mom: [in movie] Billy, do you want to walk your dog?

Billy: [in movie] No thanks, Mom. I'd rather make out with my Monroe-bot.

[Enter his dad.]

Billy's Dad: [in movie] Billy, do want to get a paper route and earn some extra cash?

Billy: [in movie] No thanks, Dad. I'd rather make out with my Monroe-bot.

[The girl from the cafe, Mavis, walks in.]

Mavis: [in movie] Billy, do you want to come over tonight? We can make out together.

Billy: [in movie] Gee, Mavis, your house is across the street. That's an awfully long way to go for making out.

Narrator: [in movie] Did you notice what went wrong in that scene? Ordinarily, Billy would work hard to make money from his paper route. Then he'd use the money to buy dinner for Mavis, thus earning the slim chance to perform the reproductive act. But in a world where teens can date robots, why should he bother? Why should anyone bother? Let's take a look at Billy's planet a year later. [The scene changes and a foam hand rolls across an empty football field.] Where are all the football stars? [The foam hand drifts across an empty laboratory.] And where are the biochemists? [The scene changes to a split screen of human and robot couples making out on beds.] They're trapped! Trapped in a soft, vice-like grip of robot lips. All civilisation was just an effort to impress the opposite sex ... and sometimes the same sex. Now, let's skip forward 80 years into the future. Where is Billy?

[The scene changes to a post-apocalyptic world. Billy is an aged man but still with his Monroe-bot and still making out with her.]

Billy: [in movie] Farewell!

[He dies.]

Narrator: [in movie] The next day, Billy's planet was destroyed by aliens. [A fleet of flying saucers destroy buildings with laser shots.] Have you guessed the name of Billy's planet? It was Earth. Don't date robots!

[A "Don't Date Robots!" caption appears on the screen and the movie ends. The Space Pope is displayed on the screen with "Crocodylus Pontiflex" written around him in English and alien.]

Announcer: [voice-over; in movie] Brought to you by the Space Pope.

(stolen from http://www.futurama-madhouse.com.ar/scripts/3acv15.shtml )