Is the definition of sexual misconduct now obsolete?
From another more modern translation (Kelly, Sawyer, Yareham) of DN31:


  "And, the mother and father so respected reciprocate with compassion
  in five ways: by restraining you from wrongdoing, guiding you towards
  good actions, training you in a profession, supporting the choice of
  a suitable spouse, and in due time, handing over the inheritance.


In the Piya Tan commentary, it says:


  Patirupena darena samyojenti, lit “they have him bound to a suitable woman,” which is said in reference to Indian society in the Buddha’s
  time. In contemporary terms, this has to be contextualized to “they
  let their son or daughter find a suitable spouse.”


Also, we can assume that adult men and women who no longer stay under the care of their parents and become independent, would no longer fall under the category of being protected by their parents. And vice versa.

So, while the definition of sexual misconduct has not become obsolete, it has become "updated", so to speak.