The court shall not consider the disability of a person seeking custody or visitation of a minor to be the sole factor in the court’s determination made pursuant to this part. If the court makes a determination to deny custody or visitation to a person who is disabled, the court shall make specific written findings stating the basis for this determination. The party attempting to demonstrate that the disability of a person seeking custody or visitation impairs the person’s ability to parent must prove that the disability is a factor, and demonstrate a clear nexus between the disability and the alleged parental deficiency.