Say straight out that it was a mistake (on your behalf) that you are mortified and that it will never happen again. Make sure it will never happen again not eve

Say straight out that it was a mistake (on your behalf) that you are mortified and that it will never happen again. Make sure it will never happen again not even by "accident" by imposing some sort of filtering/firewall/blacklist on the computer. Show them what kind of measures you took and ask for pardon. Anything more, will be kind of like defending a basically not defendable position.