Everybody makes a million assumptions about other people every day -- you can't get around it, there are simply too damn many people that you come across to consider every one as if he/she were the first you'd ever seen.
Sexism - and condescension - would imply they were saying things, or acting, in a way that said they didn't believe women were capable.
I can't tell you how often I see women crying "sexism" and making themselves feel like victims, instead of just accepting that _people make assumptions with no ill intent whatsoever_.
And I've overheard women speakers at conferences complaining about how sexist the board is, because their talks were rejected, when I was the one rejecting them because their talk proposals were off-topic, uninteresting and poorly put together...
Real sexism is vicious. Normal, well-meaning human foibles are not. Conflate the two, and you impair your own ability to function in life.
That's not offensive. That's just statistics.