Is the definition of sexual misconduct now obsolete? DN 31 says:


  In five ways, young householder, the parents thus ministered to as the East by their children, show their compassion: (i) they restrain
  them from evil, (ii) they encourage them to do good, (iii) they train
  them for a profession, (iv) they arrange a suitable marriage, (v) at
  the proper time they hand over their inheritance to them.


The definition of sexual misconduct in the suttas (addressed to men) is as follows:


  He engages in sensual misconduct. He gets sexually involved with those who are protected by their mothers, their fathers, their
  brothers, their sisters, their relatives, or their Dhamma; those with
  husbands, those who entail punishments, or even those crowned with
  flowers by another man. AN 10.176


A Commentary explains the precept as follows:


  In the case where the father is gone, or the mother provides support such as food and clothing, the mother guards [her] thinking "I will
  give [her] to a good family when she comes of age." This is called
  "protected by the mother". By the same means "protected by the
  father", etc., should be understood.


It appears quite obvious the Buddhist way of life when the Buddha was alive was for parents to arrange the marriage of their children in Indian traditional practise. This tradition would have provided little scope for sex outside of marriage, apart from outcastes and in the upper classes (who had courtesans, concubines, sex slaves from war captives, etc). 

Since the norms of society has changed greatly since the Buddha's time, 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.