I'm going to fuck an old friend's ex
Why are you choosing to do that?
Because I am desperate for sex. And not only am I desperate for new sexual experiences, but I seek for new sexual partners as well.
There is nothing wrong with seeking out sex or even new partners or experiences. But, seeking out your friends ex shouldn't be done without letting your friend know you are interested in their ex. Talk to them so they aren't caught off guard when they find out.
What for?
Letting your friend know will help to ensure that there are no problems and tension that will develop between you in the future.