People always say that, and I, frankly, don't get the connection. Were there many female characters who fit the "sex object" bill?
Well, basicly, a great number of Heinlein's characters were...pretty randy. And since most of his protaganists were male, it came across as if the women, besides being very smart, capable, bold and resourceful, were also highly decrative and posesed of a more than healthy sex drive. I'm not saying that's necessarily a bad thing (other than perhaps inflating the hopes of a teenage virgin regarding the proclivities of the average female) just making an observation.
Frankly, I would have liked to think (in those days) that all wemon were of the sort Heinlein described.