Gil has a skirt so it's obviously not just for girls
A bit more seriously, look at Proto-Arthurs design and compare it to Saber's. There are clear similarities including having something skirt-like, but Saber's ir redesigned to look "girlier".
Speaking about king needing to be male, the text feels a bit contradictory to me. How I see it, Saber needs to be male for ceremonial/customary reasons, but no one actually cares what her sex is.