I think there are various reasons for these robots to be illegal, some of the reasons i could think was: 1) because lots of people would stop trying to search for a real life partner, and instead save money for one of these, 2) the objectifying that might mean making sex robots (be it male or female) it´s a concerning matter, 3) people could use the same excuse they make with children hentai, basically "thanks, you just saved a piece of metal" of the sorts, 4) crimes could get committed by the robots, at some point it would be basically impossible to know who is making these robots, being smart enough, and having a sample of a totally working robot is all you need to be able to make another (a- apart from the materials of course).
However, depending on how it´s managed, theoretically, it would be possible to create them with minimum danger, like making them exclusively for industrial purposes, without any sex functions, emotions, or the like, etc.
However, here we have a cute yuri of a relationship between a hard worker and a sex robot, i don´t think we´ll get into these serious topics, so let´s just enjoy : ]