DDs school do mixed PE classes. When they did rugby (including contact), the boys were absolutely horrified when my DD was able to tackle them (her Dad had shown her how), outrun them, catch/throw and score tries. She is slightly built but pure muscle as she does a lot of sport outwith school.
Had she not been given the chance to play rugby at school, she would not have been spotted by the PE teacher and grabbed for the local girls team (we didn't even know there was one). She's now playing full-contact girls rugby and loving it (and she's still in one piece)!
Despite the fact that she's sporty, she was coming home after every PE class raging because the boys were so 'annoying' as any time she (or other girls) got past them/scored against them, the boys promptly started blaming each other/being incredulous that this could happen. These are the attitudes we need to challenge in sport, and that starts in early primary school, where little boys are telling little girls that they 'can't play football/have to get off the pitch'.
Football dominated playtime at the school I used to teach at. Three all - weather pitches, all full of boys playing football. When I (repeatedly) raised this as an issue and suggested that one of the pitches could be used for basketball/netball, this was dismissed as 'football keeps the boys out of trouble at playtime'.
Girls need opportunities to play physically challenging sports when they're younger, but boys also need to learn that sport isn't just their domain. Teachers/parents need to challenge the stereotypes. It would help if more coverage was given to women's team sports. I think a lot of people are surprised at the quality of e.g. women's football when they see it played.
As it stands, I'm almost coming down in favour of single sex PE lessons as by Secondary the boys' attitudes towards girls even attempting sport are often quite appalling and so the girls don't even want to try. Children should all get a fair chance to play all sports, but without the jeering that seems to happen in mixed lessons.