I'm not sure it is against the law to discriminate on sexual orientation.
We're told not to ask if they're married.
I think it depends on the state.
It's not against federal law to fire someone outright because of their orientation.
Sexual Orientation Discrimination: Your Rights | Nolo.com
Federal Law
Although federal laws protect people from workplace discrimination on the basis of race, national origin, religion, sex, age, and disability, there is no federal law that specifically outlaws workplace discrimination on the basis of sexual orientation in the private sector. (Federal government workers are protected from such discrimination.) Attempts to pass federal legislation that would outlaw sexual orientation discrimination in private workplaces have been unsuccessful to date, although more members of Congress support such a bill each year.
State Laws
There is more hope at the state level. Almost half the states and the District of Columbia have laws that currently prohibit sexual orientation discrimination in both public and private jobs: California, Colorado, Connecticut, Hawaii, Illinois, Iowa, Maine, Maryland, Massachusetts, Minnesota, Nevada, New Hampshire, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, Washington, and Wisconsin.
In addition, a few states have laws prohibiting sexual orientation discrimination in public workplaces only.