How common are romantic relationships between faculty and students? Romantic relationships between faculty members and students are often depicted in movies or books, and the student falling in love with his/her professor seems to be a bit of a cliché. But how common are such relationships in practice?
I've found a rather old survey (1982) of David L. Rowland, Larry J. Crisler and Donna J. Cox that claimed 


  Of the 184 respondents (59.8%  female and 41.2% male), over
  one-third  of  both sexes reported flirting with their instructors,
  while 46% of females  and 32% of males felt that instructors had
  flirted with them.


Flirting is, of course, is not the same as a romantic relationship. Patricia A. Rupert and Deborah L. Holmes (1997) survey earlier studies of the sexual relations between faculty and students:


  in a survey  of  807 male faculty,  Fitzgerald,  Weitzman,  Gold, and
  Ormerod (1988)  found that 26% of the 235 respondents  reported 
  having sexual encounters with students;  in a similar  study  of
  483 male and female psychology  professors,  Tabachnick, 
  Keith-Spiegel,  and Pope (1991) obtained  a rate  of 11%. Although 
  there was a considerable  difference  in these rates  no doubt in part
  due to the inclusion  of female faculty  in the second survey - 
  both indicate that sexual encounters  between faculty  and their 
  students  are commonplace  on our campus


One notable example is the marriage of Professor Philip Bobbitt (Princeton (A.B.), Yale (J.D.), and Oxford (Ph.D.)) and Maya Ondalikoglu,Columbia Law School graduate.