One of the lesser-known aspects of the ADA is that it makes it illegal to discriminate against someone because they “associate” with someone with a disability. According to the EEOC:
The ADA prohibits discrimination based on relationship or association in order to protect individuals from actions based on unfounded assumptions that their relationship to a person with a disability would affect their job performance, and from actions caused by bias or misinformation concerning certain disabilities. For example, this provision would protect a person with a disabled spouse from being denied employment because of an employer’s unfounded assumption that the applicant would use excessive leave to care for the spouse. It also would protect an individual who does volunteer work for people with AIDS from a discriminatory employment action motivated by that relationship or association.
You can still deal with excessive leave if and when it occurs, but not by blindly assuming that someone with a disabled spouse will need it and rejecting them as a job candidate.
The EEOC has made it clear that this provision applies for hiring, promotions, transfers, granting of leave, health insurance, and any other benefits of being employed; check the link for a wide range of examples. If you are tempted to judge an employee by their associations, double-check with the EEOC first – discrimination cases tend to be expensive to fight.