I'm going to have sex with my best friends ex
Where are you having sex?
We're going to have sex in a public toilet.
Attraction can be complicated but you shouldn't have sex with an ex of your best friend. It will mean the end of your friendship if he finds out.
I am willing to take that risk so I'm going to do it.
Have you tried talking to him first? It seems like you are being a really bad friend.