our bookclub is discussing MiddleSex by Jeffrey Eugenides right now. 
We are trying to remember a French film maybe 10-15 years ago (not
Louis Malle but something close) about a mother who sleeps with her
adolescent son.  Really gorgeous film.  Do you know?

I believe you are remembering either Louis Malle's film "Le Souffle au
Coeur" (also known as "Murmur of the Heart") or Bernardo Bertolucci's
"La Luna" (also known as "Luna").

Here are excerpts from two synopses of "Le Souffle au Coeur":

"This is more than just a film about teenage angst.  A teenager’s
growing attraction for girls is muddled with his intense love for his
mother, whom he depends upon for emotional security.  Inevitably, the
son-mother relationship takes an incestuous turn which ought to shock
but, because it is handled with such sensitivity, it doesn’t.

In his earlier films, Louis Malle has taken on difficult subjects
(suicide in Le feu follet, physical love in Les amants), but he
succeeds because his approach is genuinely sympathetic and
compassionate.  The same is true of Le souffle au coeur, where he
broaches the greatest taboo of all: incest."

Films de France: Le Souffle au Coeur
http://frenchfilms.topcities.com/nf_Le_souffle_au_coeur_rev.html

"Murmur of the Heart (Le Souffle au coeur)... Louis Malle's
taboo-denying tale of adolescent sexual awakening is set in Dijon in
1954, and culminates in a tasteful (!) one-off act of mother-son
incest... As most critics have been quick to point out, however,
Malle's film is hardly about incest at all, but is instead a witty,
well-observed, affectionate coming-of-age tale set against the
conventions of 1950s middle-class life."

Cinematheque: A Tribute to Louis Malle (1932-1995)
http://www.cinematheque.bc.ca/archives/malle.html

According to the Internet Movie Database, this film has also been
released under the titles "Dearest Heart," "Dearest Love" (in the UK),
"Herzflimmern," (presumably in Germany), and "Soffio al Cuore" (in
Italy).

Internet Movie Database: Le Souffle au Coeur
http://us.imdb.com/Title?0067778

------------------------

The other option that may be your film is Bernardo Bertolucci's "La
Luna." Here are excerpts from two synopses of "La Luna":

"LUNA (La Luna)... Bertolucci's controversial Luna is an unsettling
psychoanalytical study of mother-son incest. Jill Clayburgh stars as a
recently-widowed American diva who takes her teenaged son (Matthew
Barry) with her to Rome, where she is to embark on a singing tour.
There, she discovers that the youth is a heroin addict, and attempts
to wean him off the drug by offering herself to him sexually."

Cinematheque: Masterpieces of Italian Cinema
http://www.cinematheque.bc.ca/archives/italm.html

"Bertolucci stirred up controversy again with "Luna" (1979), which
featured some unsettling incestuous interplay between an American
opera diva and her teenage son."

Daily Bruin: A Director's Strategy
http://www.dailybruin.ucla.edu/db/issues/96/10.24/ae.bertolucci.html

My Google search strategy was greatly simplified by the fact that I
recognized these two films as likely candidates from your description.
To find plot descriptions which would verify that one of these is
likely to be the correct film, I used these Google web searches:

Google Web Search: "malle" + "le souffle au coeur"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=malle+%22le+souffle+au+coeur

Google Web Search: "bertolucci" + "luna"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=bertolucci+luna

I hope this helps! If neither of these is the correct film, please
request clarification before rating my answer, and I'll be glad to
resume the quest.

Best wishes,
pinkfreud

Thank you!