Sports and athletics do not necessarily need to be equated with down-n-dirty, shouting, high-impact, dare I say *masculine* activities.
There are "feminine" sports out there with very fit and devoted lady athletes. Take ballet, for instance. That's as 'girly' as it gets, but it's still a difficult and strenuous discipline. Or other things, like gymnastics, dance, yoga...even cheerleading qualifies. I also don't find anything particularly manly or intimidating about girls that do casual workouts like jogging or bike riding.
I suppose when the "typical guy" thinks of an "athletic girl", he envisions hairy-pitted members of the local lesbian softball team. It's pretty sad that men in general have yet to grow out of the mentality that feminine = physically weak and helpless. So what if a girl likes flag football? She can still wear skirts and makeup and gossip and cry when her feelings are hurt. It doesn't mean she's going to beat you up, and it also doesn't mean you have lost your masculinity if she happens to be a more talented player than you.
I was a swimmer for 6 years, and thanks to 3-hour-a-day workouts I have a lean, toned body. Yet, I am still irrepressably feminine. No guys, physical exertion will not deplete a woman's supply of estrogen. While I do enjoy being healthy, I am not particularly obsessed with any sport nor am I going to let competition circumvent more important priorities.
Incidentally, I find that it's men who have a problem with that.