It does it even if you've added your ssh key?! Really?
So edit .git/config and change repo url from https to ssh.
It should look like this
url = firstname.lastname@example.org:your_login/your_project.git
If you don't know the address then go to your bitbucket repo page and check SSH address on the project's Overvier tab.
Don't forget to set up your name (bitbucket login) in [user] section. Refer git manual or just type
$ git config user.name your_login
$ git config user.email your_email