FS (and others), you are very good at looking at things from your perspective, but not so much at looking at things from someone else's perspective. Imagine being told your entire life, implicitly if not explicitly, though often both, that you're just not good enough. That you can't measure up to those boys. They are more athletically gifted than you, smarter than you, and that your job when you become an adult will be to make babies and raise them.
And before you say that surely we as a society have progressed beyond that, let me assure you as the father of three girls, those messages are still there, loud and clear.
Now suppose that you, as a teenaged girl, have found something you are good at and that you enjoy, in this case wrestling. The problem is, there are girls leagues. What do you do?
As Boone said, it's a performance issue. If they are good enough to make the team, so be it. If there isn't a women's team, why not allow them to play? And if the boys don't want to fight them, I get that too. Disallowing them from playing only serves to reinforce the stereotypes that girls aren't as good as boys at athletics.
(I know. Girls aren't as good as boys at SOME athletics. It's true.)
Your second point about men competing in women's leagues is absurdly simple, and I'm surprised you even threw that out there. There is a clear hierarchy in sports in our society, with men's sports being at the top, and women's sports being below. We need to give the women something at which to aim, not give the men something to fall back upon.
Yes, some gal will end up getting hurt. I suppose you think that men don't get hurt in these events because they are tougher/stronger?