What should I do if I forget the Basmalah before intercourse and remember it during or after it?

Remembering Allah before intercourse is recommendable, not obligatory. If someone forgets, there is no sin upon them, based on Allah's saying: (Our Lord, do not take us to task if we forget or make a mistake) and the Prophet's (peace be upon him) saying: "My Ummah has been forgiven for mistakes, forgetfulness, and what they are forced to do." If one remembers during intercourse, they should remember Allah Almighty.