Sure, but lets not pretend they are "defining what femininity" is for themselves.
Yes. Let's not let them talk about it. There's a woman here telling you that you're wrong, therefore she must be crazy and thus ignored. Listen asshat, when I say defining it, I didn't say it's easy. I didn't say that it was right. I didn't even comment about the social acceptability of it. What I said was: It's up to me to say what it means. And if I decide it means running around in a miniskirt and pom poms, well... screw you. If I decide it means putting on a burka and hiding my face and body from the world, screw you too. In fact, if I decide it means nothing at all, you guessed it: Screw you.
Your opinion, sir, is simply not relevant. If a woman has the choice, then she is free. It is when we stop having choices that there's a problem, and your attitude, expressed by putting it down to "as a society" to avoid taking responsibility for it, is what causes the capacity to choose to diminish. Women have bodies. They're not shapeless automatons, but beautifully curved, soft, and all of that. And why shouldn't they be allowed to revel in that?
It only becomes a problem when other people's sick notions of what 'normal' should be draws others into the kind of behaviors you describe. And nobody is immune to that, not even you, Mr. I-Ask-Myself-Every-Morning-Who-The-Tiger-Is. We all have to deal with our own body image issues, men, women, human. That's just how it is.
But as long as you have the freedom to choose how you face those circumstances, it's all good. When you start demanding others not have those freedoms because you feel you're "saving them from themselves", well then Sir, you are part of the problem.