I'm not denying or disagreeing with your experiences oh and giles. But in dds case I'm really glad it was mixed.
It was a small primary and we live semi rurally, so separating at primary age would have just very much limited her chance to play, both in school or just when playing out.
And imo it was a positive for all of them, dd and another girl because they didn't feel they shouldn't intrude male dominated activities, the boys because they learnt that regardless of what society said the girls were as good or even better at 'boy' stuff. The boys she played with never had anything negative to say, and on more than one occasion jeered anyone who did try it.
She's sporty but at secondary level the type who makes the team, rather than being particularly talented. And in fairness that's more due to fitness, and at primary age her ability to plan and play tactically. So not a positive experience because she's amazing.
And I have no doubt that if she hadn't grown up playing alongside them, at 14 she and one or two other girls certainly wouldn't still be able to join in local mess abouts with local, mainly teen lads. All very well if you live somewhere more populated/ better public transport, but not at all practical for us to have to drive and plan in advance every time dd fancies kicking a ball around.
Not that this will last, but she's very tall and fast, if slight. So until the boys start bulking out and gaining an advantage she is still treated as an equal.