#1) I reject the assertion, held by Johnson, that it is not permitted for women to play team sports (provided it's against other women and not violently aggressive ... like football ... and there's no public immodesty). There's ZERO Church teaching to back this up. In fact, Pius XI stated that such sport in "public" exhibition could offend modesty. So, in other words, the sole considerations is modesty, and not the sport itself. He implies that it is permitted for them to play in a more private setting.
#2) With regard to modesty, there are degrees of immodesty, and as is the case with all things venial, if the matter is not intrinsically evil, then a proportionate reason could justify a certain amount of immodesty. Grave immodesty would require a grave reason. Let's say a woman has been kidnapped and is being held captive unclothed somewhere. She escapes and runs out in public without clothing on. Grave immodesty justified by a grave reason. So too, venial immodesty, can be justified by a proportionate reason. The slight immodesty of showing one's knees to be able to properly play a game without risk of injury can be justified.
To answer LastTrad's questions:
Please drop the arrogant "you have been told" language. I have been old nothing. We do not know how those outfits were designed, how they react to the movements of the game, or what they have underneath. Yes, if they do fly up to reveal their panties, for instance, then that could rise to the level of being gravely immodest. But none of us know the details of how this would work. I am going simply on just what I saw. If the girls would go out in public, say, to a grocery store, with those outfits, I would consider that venially immodest. But, assuming it's not the case that it becomes gravely immodest during game play, then there's proportionate reason for them to dress like that.
If the other teams do dress in a gravely immodest fashion (we do not now what kind of league they play in and what their rules are), then indeed it would be inappropriate for men to attend those games. Whether or not this would constitute a near occasion of sin for them, it would still be scandalous for a man to watch such displays, and therefore inappropriate and at least venially sinful (depending on the degree of scandal given).