You're making the mistake of thinking that dresses are formal attire. I'm not a woman, but I don't think that's a valid assumption.
In any case, the point here is that men's reactions to women in dresses often cause distress and make women feel uncomfortable in technical settings. That's frankly ridiculous, since (as far as I understand) many women like to wear dresses and don't feel that they're formal. Maybe it's due to a misunderstanding? Men labor under the misapprehension that dresses are formal attire when they're not, and that causes the disconnect, etc. etc. Doesn't mean we should just accept it -- let's correct the misunderstanding. :)
Anyway. Maybe it's worth asking the women you know.