Or maybe it's that it takes a certain sort of confidence and aggression to make it big in science and Watson is reading this as 'unwomanly' and therefore negative. I don't think it's true any more that women have to be extremely aggressive to reach the top in academic science, but certainly in the past, in Watson's time, this must have been true. By definition any woman he met in the lab was probably fairly driven. These qualities then could have become misinterpreted in his mind, then settled down into a 'rule', which in turn could be imposed on every new female he met until the prejudice was set in concrete. He maybe wanted his women 'girly' (flirtatious, deferential to himself, giggly) and when these qualities were not met, he put a different spin on them.