At my school the favorite game in gym was 'kill the guy with the ball'. Basically everyone tried to get the guy with the ball, and that guy tried to hold onto it as long as possible in a vain attempt to prove his macho. The battle of Waterloo was won on the playing fields of Eaton, and sports are just war for boys under another name. Dodge ball is a lethal game, basketball is very physical, and out here in Cali we played volleyball for blood, go ahead, stop that spike with your face - Have Some!. And that kind of aggression is not 'tolerated' it's actively encouraged. "Boys have to work out that energy somehow." And sports is our cultures way of letting the boys blow off some steam. Oh yeah, it's also were a lot of personal scores are settled.
So I don't care if there is no tackling in flag football, it's still a totally violent and physical game. I'm betting that I wouldn't even have to tackle you, I could body block you right into the ground, if not clear into next week (really, how tall are you/what's your weight? When I was in HS I was 5-9, and about 170, and it was a lot of muscle - if I was running at you at full speed could you stop me? And I was one of the smaller guys, god help you if any of the real jocks are covering you). I played full contact football my freshmen and sophomore year and I can assure you that the flag football games in gym were much rougher than the full contact, because in full contact you have pads and helmets and all sorts of special equipment, in gym you have a shirt, shorts and a jock strap, not much protection there.
And you need to understand that along the same lines as the playing fields of Eaton, boys are encouraged, socialized and peer-pressured into taking those games, in gym class, or pickup baseball games, VERY SERIOUSLY. You do all that 'playing to win' and giving 110% at all times, taking one for the team, playing through pain and all the other sports cliches. It's not like you have to endure a shaming if you lose, and if you take it personal that's bad too - you're supposed to 'leave it on the field' - but for the time you're playing, you're playing to win at pretty much all costs or you'll be the least popular boy out there because you'll be 'making them all look bad'. And being the guy who is always picked last - or worse when the team that has to take you gets an extra good person to compensate - well that is a public shaming (and there is one guy like that in every class, count on it, and make sure its' not you). Because if your 'that guy' you'll get a public shaming every single day.
If you are out they are going to make a point to make sure that if you want to be a guy so bad, then you better guy-up.