What is the meaning of Touchstone's speech to Corin ("That is another simple sin in you...I can't see else how thou shouldst escape.") in Act 3,...

First of all, you must remember that Touchstone is a
clown.  Many of his speeches are spoken mainly to be funny.  This speech is there
largely for this reason (though it is also meant to show the differences between a court
person like Touchstone and a country person like Corin).  Basically, what Touchstone is
saying is that Corin is immoral because his job involves being a pimp for
sheep.

In this conversation, Touchstone has told Corin that
Corin is damned because he is not from court and has no manners.  Corin says he will
"rest" and Touchstone wants to know how he can rest when he knows he's going to hell.
Touchstone tells Corin that even his job is sending him to
hell.

This is the point of the lines you ask about. 
Touchstone says that Corin's job is a "simple sin" meaning a sin that arises because
Corin is "simple," (ignorant).  He says tha Corin's job involves bringing male sheep to
mate with female sheep.  This makes him like a pimp bringing men to prostitutes. 
Therefore, Touchstone says, Corin's job is evil and he is damned.  He will only escape
going to hell if the devil does not want any shepherds in
hell.

So, this is a comic speech that also shows
differences between the elites of the court (seen as bad in sylvan plays like this) and
those of the common people (portrayed as good and sensible).