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?