Because it keeps you focused on her.
Sex binds you. It does not free you from her.
The more sex you have with her, the more you see her as your female partner.
You develop attachment from her.
Yes! Sometimes, you will meet and you will both be in the mood for it.
It might happen.
Now, observe what happens just after that?
What happens in the week after?
Do you get this intense feeling of emptiness?
Do you know why?
Because when having sex you invest yourself in her.
You invest yourself but get back nothing.
You end up trapped between conflicting directions.
You spend the next few days or weeks trying to refocus on gaining back your power until she shows up and steals again your newly gained emotional freedom when you have sex.
Don't go that way!
It is a dead end!
I know it is tough but if you want to have sex, focus as much as possible on new partners rather than on your ex.
That way, you will speed up your power recovery!
To your power!