I can see how it is sexist for this to be a problem with the bystanders, but to me this suggests a problem with the way in which these bystanders are viewing the girls. It's likely that offense was taken because a girl in a bikini inspires sexual thoughts or possibly inappropriate thoughts to these people. If the same feelings are not instigated by the males, then less offense is taken.

I suppose that's one way that there is a difference; the girls are perceived as a sex symbol, while the boys are continuing the sexist tradition of masculine sports obsession. In Europe, breasts (both male and female) aren't nearly as sexualized. The way they are perceived is the problem.

The sports bra idea is a good compromise, i'd say. But it's totally unfair.