"There is no federal law preventing discrimination on the basis of sexual orientation."
I believe you are right. There are laws that prohibit discrimination against "sex", and some states have limited laws to protect gays from discrimination... but not discrimination against heterosexuals.
http://www.fairhousinglaw.org/fair_housing_laws/federal_laws.htm Federal Fair Housing Laws
Fair housing laws provide a critical way to deter and counteract housing discrimination. Follow these links to learn more.
Fair Housing Act
Other Federal Laws
State Laws
The major federal fair housing law is the Fair Housing Act. Title VIII of the Civil Rights Act of 1968, as amended, prohibits discrimination in the sale, rental, and financing of dwellings, and in other housing-related transactions, based on race, color, national origin, religion, sex, familial status (including children under the age of 18 living with parents of legal custodians, pregnant women, and people securing custody of children under the age of 18), and handicap (disability).
Complete Text of Fair Housing Act >>
The federal law does not cover discrimination in housing based on sexual orientation, but fourteen states plus the District of Columbia have passed laws that provide some form of civil rights protection for gay, lesbian, bisexual and transgender people.
STATE BY STATE: Civil Rights Laws Prohibiting Discrimination in Housing
Fair Housing-Related Laws
Title VI of the Civil Rights Act of 1964
Title VI prohibits discrimination on the basis of race, color, or national origin in programs and activities receiving federal financial assistance.
More on Title VI of the Civil Rights Act of 1964
Section 504 of the Rehabilitation Act of 1973
Section 504 prohibits discrimination based on disability in any program or activity receiving federal financial assistance.
More on Section 504 of the Rehabilitation Act of 1973
Section 109 of Title I of the Housing and Community Development Act of 1974
Section 109 prohibits discrimination on the basis of race, color, national origin, sex or religion in programs and activities receiving financial assistance from HUD's Community Development and Block Grant Program.
More on Section 109 of Title I of the Housing and Community Development Act of 1974