This is one of those thing in which there are an awful lot of misconceptions.
Sentience and sapience are not magical things that are attached only to humans or life forms. Our bleeding edge experiments in robotics have been demonstrating the further along we get in emulating human behavior the less difference we find between bio and mechanical. Before the end of the century, we're going to see bots with whom we interact emotionally the same way that we interact with our fellow humans.
At the same time, sexual attraction is a very complex thing. Much of it has to do not with emotional connection but the oh-so-subtle cues that partners relay to each other unconsciously to suggest we would breed strong children. Female pheromones relay they're ovulating. Male pheromones communicate a robust immune system. Both communicate a genetic formula that is analyzed by the other, and if both after this process decide its a good match then the mating process continues to copulation (and when possible, establishment of a family). It's likely that even a sexbot that's obviously botlike will become preferred to human partners if we are able to emulate the pheromone system [http://whyevolutionistrue.wordpress.com/2009/08/26/orchids-mimic-alarm-pheromones-of-bees-to-attract-wasps/], and successfully make an ideal cocktail for anyone.
There is the uncanny valley, and one solution is to make obvious sexbots [https://www.google.com/search?q=sexy+robot&safe=off&tbm=isch]. In fact I think this is more likely since that way they'll seem less a competition to human partnerships. (They will be, but they'll seem less like it if anyone can easily recognize a robot over a human partner)
The problem with *! [https://www.google.com/search?q=sexy+woman&safe=off&tbm=isch]) is not just the complication of making one realistic, but then we'll be able to make them hyper-realistic, so that they are even more attractive to us than other humans. Just as Playboy magazine taught a generation of young men (or two) to seek out a narrow bandwidth of female body types as ideal, sexbots may make it impossible for humans to compete at all.
This might kill accidental births entirely, and reduce our population growth accordingly (since accidental births are a near-guaranteed recipe for dropping your economic and social class) Of course, by then, we can just testtube all our babies and birth them in artificial wombs [http://en.wikipedia.org/wiki/Artificial_uterus], so it may not matter.
But the thing is, human partnerships have that tedious aspect of having to deal with another person, and interact with their preferences, opinions and rights. If you're in contention with your own robot, that can be fixed. If you're in contention with a human partner, you have to compromise, and sometimes those conflicts are unresolvable. Because of power of the human veto, Sexbots can always be negotiated around, where humans cannot.
As misanthopic as I am, I'm disinclined to inflict myself permanently on someone else, and having a robot that provided for my social needs when I'm feeling grouchy (and kept me in practice of being polite and sociable when I'm particularly reclusive) would actually be doing the rest of the world a service. So yeah. I'm there.
Of course, if you like your sexbot, you will LOVE the latest model...
238U
. On the other hand, however, yeah...creepy as fuck. So, no thanks... I'll pay for sex the way men all over the world have been doing since time immemorial - dinner, movies and jewelry 
