Model Rule 1.8(j) was adopted a number of years ago to regulate possible sexual relationships with clients. It states that "a lawyer shall not have sexual relations with a client unless a consensual sexual relationship existed between them when the client-lawyer relationship commenced."
The topic is now back in the news because a few days ago it was reported that California is considering adopting the Model Rule's approach to the question (as part of an overhaul of the state’s disciplinary rules). The current rule in California bars lawyers from coercing sex with a client or demanding sex in exchange for legal representation. For more details and links go to NPR, or the ABA Journal. For a comment on the proposal go to Simple Justice.
The topic was also proved difficulty in Texas when it considered changes to the rules back in 2010.