Thighs are stronger than calves, and you can get a tighter grip, as well as have a higher chance of breaking things. Peter was intending to disarm, Natasha was ready to kill. Natasha is a trained assassin, and Peter is a student who works for a newspaper.

Given their backgrounds and experiences, it would be UNequal to have Peter using skills and disarming tactics that Natasha was trained to do so.

So yes, this whole equality thing is great.