I would be careful in using that lease clause. It appears to violate Washington State Fair Housing laws as it applies just to single individuals only. That's a violation of Marital Status, which is protected by State law. It might also violate SCRA Act and the Domestic Violence Act. Such a lease clause may also be in violation due to a disparate impact against single applicants.
Defending such a lease cause in Federal court or before an Administrative Law Judge would be next to impossible and expensive. As was stated " joint and several covers that in three words" and applies to everyone both married and single, thus no possible Fair Housing violation.