I'm going to break a friendship without warning.
Do you want to elaborate on that?
We had sex two times and then coldly left.
You should treat people the way you want to be treated. Give people the respect of letting them know why you don't want them in your life.
What if I don't give a shit?
It is still wrong to abandon people without an explanation. You should let them know so you have a chance to save your friendship even if you have no romantic compatibility.