Sex will not cause your tubes to become untied. What causes it is your tubes after being tied can end up touching each other in your body. After touching for a while they will begin to grow back together. Your body just thinks its repairing itself. But then it is possible to get pregnant again. There is a procedure that when they tie your tubes they cut a section out to keep them from growing together but many places don't like doing that because it can never be reversed once its done.
