I was not able to search on
www.srigranth.org as was getting timed out.
I found the following excellent article and the relevant extract from it:
http://allaboutsikhs.com/books/kjs/06.htm
Guru Nanak also condemned the idea of impurity of a woman in the days of her menstrual cycle. Guru Nanak said there is no impurity in it. It is a natural cycle. Actual impurity is in the mouth of a person who tells lies after lies. Impurity is due to bad qualities and not due to natural bodily function (SGGS., p. 472).
Then he condemned the idea of impurity after the woman gives birth to child. There was a system to keep woman isolated from the rest of the family for some days after the child’s birth. She was not allowed to touch anything because her touching anything would render it impure. This impurity was called Sutak. Guru Nanak raised his voice against this evil practice. He said everywhere reproduction is taking place. Even the cowdung-cakes, used to cook food, are not free from it. The insects are reproducing, then, there must be impurity in fire also. None of the things we eat or use otherwise is free from life, which is multiplying every moment. He told that actual impurity is due to evil thoughts of mind. In Asa ki Var he says:
If impurity attaches (to life’s birth),
then all, all over, are impure.
In the cow-dung and the wood too is the life of worms.
As many are the grains of food, not one is without life.
And, is not water life that brings all to life?
How can then we believe in life’s impurity,
when impurity is in our bread?
Nanak: impurity goes no otherwise save by being wise.
Further the Guru tells about the actual impurity:
The mind’s impurity is covetousness;
the tongue’s impurity is Falsehood.
The impurity of the eyes is coveting
another’s woman, beauty and riches.
The ear’s impurity is to hear and carry tales.
Nanak: even the purest of men, thus bound,
go to the city of the Dead. SGGS., p. 472
On the basis of these ideas he described Sutak as an illusion of mind:
All idea of impurity is illusion of mind,
which attacks those who are in the duality of Maya,
The creatures take birth and die through His will through
His Will one comes and goes.
To eat and drink is pure:
For the Lord hast Blest us with these in His Mercy.
Nanak: they who Realise the Truth through the Guru,
to them Impurity sticks not. SGGS., pp. 472-73