You know - you bring up and interesting point. What exactly is the difference? Because the robot/doll has a face? You are still mainly using either a hole or shaft for fun (depending what you are doing.) This can be realistic, or purely functional toy wise. But your toy serves the same purpose as the doll/robot. If you aren’t using it solo, then because it has a face it your partner is more likely to get jealous (as some do with regular toys).
You say it would be for “servile” reasons, but I have seen a couple Real Doll documentaries and many of them have more of a faux loving relationship. I mean, I agree, some of it is pretty creepy. But on the other hand, wouldn’t it be a bit creepier if you treat something with a face like you treat a dildo or a fleshlight?
Though I suppose if the Robot was advanced enough to be AI that would open a whole host of ethics. But if it is basically an adult Teddy Ruskin telling you he/she loves you, it is just and features for the same basic purpose?
So, yeah, part of me agrees with you, and part of me think its just a sexual quirk that we just don’t really understand - similar to how some people are with BDSM or gay sex or dressing up in a Boba Fett Costume and tying up Han Solo because of their lack of exposure and how it just seems to foreign to what they are into, they find it off putting.