it's unprofessional and unethical. Part of the essence of teacher-pupil relationships is that the teacher is trusted as an adult in the place of parents (in loco parentis is the legal term).
Moreover with older students it opens the professional to accusations of non-impartiality.
And it goes on all the time. And it didn't stop the career of Chris Woodhead, ex-head of Ofsted, who did a sixth former he taught while maried to another woman and teaching at the school.
Double standards in England? Oh, no, never.