It might be helpful to remember that poly relationships can be super intimidating and triggering from the other side.
If Sally asked you from the get-go "where is this going?" she may have been dealing with feelings for you long before you asked her out. To be asked out by you while you're committed to someone else puts Sally is the "2nd best" category, which for some people is just unbearable. Her curt email "I don't condone poly" was clearly an act of her expressing that pain and, really, I don't even think it was intended as a judgement of you/poly as much as it was a way for her to act out....