We are human, so we will make robots like ourselves just as we do with dolls and sculptures and really most of the history of human creativity. And as we make robots more human, we will assign them gender because this is how we see each other, so it isn't necessary to view them as sexless objects. I agree, however, that the stereotypes seen in most robots are ridiculous. But I guess it's because sexy female robots have a pretty niche appeal, mostly single, young, male sci-fi fans who have stereotyped views of womanhood and attractiveness.
I've always been a fan of robots, and I think it's possible to give them very clear gender differences in a way that is not stereotyped.
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"If knowledge can create problems, it is not through ignorance that we can solve them." - Isaac Asimov