(Many new posts whilst composing...)
How much is sexual dimorphism relevent in the sitting geometry of the hip and thigh areas?
Wider hips allow more upper thigh to comfortably lie close together without spread at the knees, the 'thigh gap' seems to be a sought-after phenomenon (at least recently) meaning it's the traditionally leaner knees that dictate the spread (or, in this case convergence) angle. Men have narrower hip-joints and may well have thicker thighs (fat and/or muscle, depending on lifestyle) already pressing together.
Depending on how I'm sat, it isn't always possible to get my knees to touch, I have discovered whilst consciously thinking about this issue as I'm sat at a window seat and the rapidly filling bus puts someone else on the aisle side. (Strangely, though, I can do this right this moment. It might depend upon things including the actual trousers I'm wearing at the time - perhaps the feeling of straining certain seams or creasing other bits of fabric into my flesh without the easy ability to fidget those tensions in the material away round the curve of the joints.)
The male variety of equipment between those thighs tends to find a place to be, in my experience, so it's not so much part of the topological problem except maybe with badly fitting underwear, but I can't personally compare and contrast with those who have to manage with the alternative (and possibly their choice of monthly products being an issue. Ditto there are two different forms of potentially being hot and sticky.
Also, socially (and sartorially) girls are not encouraged (actively discouraged) to spread their legs whilst sitting, so maybe us men have never quite had the imperative to try to reduce our natural divergence of thighs (and the monotubular nature of skirting likewise a restriction, if pleating or slitting1 isn't a major feature to allow movement...), even when we aren't trying to display our 'manliness' on whatever conscious level.
1 In the last few minutes I have learnt more new things about skirts and dresses than I previously already knew, as I tried to qualify my existing ideas by looking at everything from hobble skirts to Empire silhouettes in a bit of crash research on the subject.