Interesting. I work full time and always have done. I have, in the past, given up afternoons (taken annual leave) to help listen to children read (the teacher said "It's always the working Mums who give up time to help, we don't seem to be able to entice the SAHMs in", I have suffered sports day, avoided harvest festival and other dreary stuff in the church ("Of course I was there! I was next to X, behind the pillar near the front of the church at the right, that's why you didn't see me!").
I bake cakes for school cake sales - my muffins are legendary. 

I help out every year, all day, on a variety of stalls, at the school fete.
I have been asked - as ilikerain has - why I had children.
Why doesn't anyone ask the fathers these questions? Why is it always the mothers who get asked, and it is assumed that the fathers won't be doing any of these things.
My children come home to a parent after school. But it's not me. What is the matter with that?