Freedom of Association is not explicitly listed in the First Amendment, but the Supreme Court has never-the-less upheld association rights in expressive organizations and for intimate associations, such as the family and more broadly in private social clubs.
The State of California continues its attack on Craigslist and Roommates.com trying to make these organizations liable for California Fair Housing Law violations when they publish a classified ad that breaks the law. In short, it is illegal in California (and some other states) to advertise for a roommate who is a specific gender or race or religion, even if there are strong compatibility reasons for doing so (As in most states, it is A-OK to discriminate against smokers).
I won't get into the whole legal argument about these listing services, except to say that it is absurd to hold third parties accountable for other people's speech. I want to ask a more general question. How do laws that prevent me from choosing a roommate (however I want to) pass constitutional muster? Taking on a stranger for a roommate is a scary proposition, especially in states like California that make it well nigh impossible to evict someone once they have moved in. Short of marriage, it is hard to imagine a more intimate relationship -- in fact, many roommates probably see more of each other than some spouses. On average, most people are probably not a compatible roommate for me.
Beyond this, most of the people who run afoul of the housing law do so with their speech, not the actual selection of a roommate. Most fair housing complaints are against people's advertisements or public statements. This strikes me as a double violation - the banning of speech about my association preferences.