First of all, I assume you mean liable and not libel.
Second, what are the liability risks? Sexual Harassment comes to mind, but this would be primarily focused at the individual who accessed the adult content, unless the company elected to not take action to stop the harassment once reported. Negligence or "fostering an environment" are the most likely risks.
Third, how do you mitigate these risks? The primary method is to enact a company policy prohibiting employees from accessing (or possessing, distributing, or creating) adult materials in the workplace or using company resources, and then require each employee to sign a letter of acceptance of this policy as a condition of employment.
What else is required beyond the policy to mitigate liability? Enforcement. Not filtering, but appropriate and consistent disciplinary response to each infraction. Failing this opens a company up to liability for failing to protect its employees from sexual harassment.
Filtering is an added factor that demonstrates a company's commitment to enforcing its sexual harassment policies. But, as we well know, filtering is not difficult to bypass. Just because an employee can find a way to circumvent the filtering does not mean the policy forbidding this activity is no longer in effect. It is the policy itself, and consistent disciplinary action for offenders, that mitigates the risk of liability.