Googling and looking online at fair house law and so forth have not yielded any information on the conditions needed to be met for a group to be protected under fair house and considered a protected class.
There has to be a law that prohibits discrimination on that basis. Here is a list with mentions of the relevant law. Every state has some such laws. There is generally some agency that regulates the activity, so the Office of Fair Housing and Equal Opportunity within HUD creates and enforces various regulations in their sphere. So pregnancy is not a protected class with respect to sale, rental of financing of housing, if FHEO online documentation is correct (pregnancy is protected on w.r.t. employment by employers with 15 or more employees). State regulation is particularly relevant for insurance since insurance is tightly regulated at the state level, and a state Human Rights Commission is often in charge of regulation and enforcement. There may also be city regulations, so in Seattle, political orientation is a protected class.