Rather than "Why are assistants conflated with women?" a more interesting question is "Why do assistants tend to be women?" (If my experience is average.)
I don't really know the answer, but I hope it's not a controversial question, since I'm genuinely curious about the forces involved.
Lots of reasons, but a major one is that until relatively recently that's one of the jobs they were permitted to have: http://scholarship.law.cornell.edu/cgi/viewcontent.cgi?artic.... In the legal world, for example, law firms openly rejected top applicants solely because they were women well into the 1970s.
People tend to think of that time as ancient history, but it's not. C and UNIX were developed in the late 1960s and are still the bedrock of the Internet. Why should we expect social phenomena would move faster than computing technologies?
That is, he's progressive enough to hire non-whites, but still doesn't think men can be secretaries. (In fairness, they probably wouldn't have wanted the role anyway.)
I don't think any of those observations are very controversial.
The continuing problem is this hasn't changed much at all in the last half century. For example, you will not find one unattractive receptionist at any VC office on Sand Hill Rd.
It's not, really: displays of health, wealth, leisure are pretty much common. Youth and fertility for women. Status and wealth for men. Body symmetry.
My layperson's understanding is that some of the specifics can be contextual to a particular time and culture, but the principles are universal, and so are a good number of the specifics.