When first it began, mankind developed these systems to do things which could not be done by man without great risk. Or things which were required to be done with great speed and equal results. But as time progressed, technology improved and prices fell, it became common to see such robots used for frivolity.
Ultimately it was the sexbot which became the fall of man. Machines so similar to real human beings that you could not tell them apart either by look, touch, smell, taste or mannerisms. Half machine and half biological in design they utilized human genetics as their map for diversity, meanwhile weeding out undesirable traits the genetics possessed. Human beings had used this same technology in labs to ensure their offspring would be born with traits deemed more desirable.
As the sexbots were of similar fashion it was soon the desire of all young men to possess such creatures with amazing beauty and impeccable bodies. No matter their shape, intellect or inability all young men would not be turned down by such creatures.
As their male counterparts no longer desired interested in them, the females of the human species also turned to sexbots for physical affection and emotional support, as well as to other female humans.
The machines were nearly undetectable. A war could not be fought if an enemy who was not really an enemy could not be found among the populace. One could not detect who was man and who was machine. A true blended society exists.
It was only a matter of generations before pure human numbers dwindled. The population of machines formerly referred to as sexbots were in fact of greater number, Sooner or later humanity found itself nearly extinct. Its legacy existing in the purified genetics of the new blended race that now ruled the earth.
So this story is about humanity evolving into machines by creating machines similar to themselves. Even so far as using human genetics to create sexbots which are covered in human flesh and possess human-like characteristics and diversity.
Lets think about this for a minute. You’re a horny young man. You exist in the future where robotics technology and artificial (I hate that word) intelligence is far beyond what we know of today. Where you go to high school there are some really hot girls, but you’ll never know them because you aren’t hot yourself. However you can go out and purchase a sexbot which is just as hot, if not more so, then any of the girls at school. You can love her up as much as you want and she will totally accept you and want every part of you because she is designed to adapt to your personality traits. At some point, real girls become unimportant to you. They don’t want you and now you no longer need or want them either. Eventually this trickles through the entire human race and pure humanity becomes extinct because few human beings procreate with other human beings any longer.
If at first this sounds like science fiction, remember that humanity is a very malleable species. What one generations sees as heresy the next generation grows up with accepting it as just another part of reality. My five year old son for instance never knew a world that existed without cell phones or computers. Given enough generations (possibly only two or three) that grow up in a world where robots that are just like humans exist, they will come to accept them as just fellow co-inhibitors of the reality they live in. Eventually the superior traits of this new race will win out and humanity will find itself extinct. It doesn’t need to be a bloody war. All it takes is a slow and gradual process of breeding out the human race.