I'm going to sleep with my best friends ex gf
Why would you sleep with your friend's ex?
Because I feel like it. I mean, they are no longer together anyway
I think that would be a bad thing for you to do. It's generally frowned upon to sleep with a friend's ex. This could cause a rift between you and your friend so it might be best to not do it.
What? You really believe it's wrong to sleep with your best friend's ex?
How do you think your friend will feel if they find out that you slept with their ex? It would probably be a good idea to try to find someone else for an intimate relationship so that you don't end up causing tension between you and your friend.