Skirts or dresses aren't a problem when going through security. Just think of all us Flight Crew who often choose to wear our skirt and blazer or dress. By regulation, the length of our hemline must fall at the knee or just below. If it were any shorter it would be uncomfortable to sit without having to pull it down all the time. Fortunately, we don't get to sit for much more than takeoff and landing, so it's not a problem. However, sitting as a pax, a skirt of that length might get uncomfortable. Bare legs can get chilly, too.
From a safety standpoint, pants are best. (Kind of a contradiction here when we crew wear skirts, I know.)
But it's true, pants will give you more protection in an emergency, esp firefighting, esp if they are natural fibres.
Pantyhose, in a fire, will melt onto your skin which is very painful to remove and treat the injury.