The critical mistake is assuming that this is sexist. It's not. It's basic pattern-seeking, a simple and harmless assumption. Getting all up in arms over it is worthless!

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.