Recently, the Madison City Council passed an ordinance doing just that:
In what is believed to be a first in the United States, the Common Council of Madison, Wis., has voted to amend the city's equal opportunities ordinance "to add nonreligion as a protected class."[...]The new legislation inserts the words "or nonreligion" after "religion" in several sections of the city of Madison's legal code.Surely that's a typo that "gender identity" appears twice? (I suspect one of those is "gender expression.") What does seem surprising is that this would be the first ordinance of its kind. Especially considering that the Freedom From Religion Foundation is based in Madison!
For example, here's how the city lists its protected class:
"Protected class membership means a group of natural persons, or a natural person, who may be categorized because of their ability to satisfy the definition of one or more of the following groups or classes: sex, race, religion or nonreligion, color, national origin or ancestry, citizenship status, age, handicap/disability, marital status, source of income, arrest record or conviction record, less than honorable discharge, physical appearance, sexual orientation, gender identity, genetic identity, political beliefs, familial status, student, domestic partner, or receipt of rental assistance."